State Sales and Use Tax Nexus Rules Every Multi-State Business Should Know

7/21/2026

The most expensive sales tax mistake a CPA can make on a client's behalf takes about four minutes.

The client says they have been selling into a dozen states for three years and have never registered anywhere. The CPA, wanting to fix it, goes to the state's website and registers them.

That single act can eliminate the client's eligibility for the program that would have limited their exposure — and can convert a negotiable historical liability into a filed, dated admission of a start date the state now knows about.

Do the analysis before you touch a registration portal.

Two Questions People Merge Into One

Nexus asks whether a state may require you to collect its tax.

Taxability asks whether the specific thing you sell is subject to that tax in that state.

They are independent. A business can have clear nexus in a state and owe nothing because its product is not taxable there, and can sell an unambiguously taxable product into a state where it has no nexus at all. Answering only one of them produces confident, wrong conclusions in both directions — which is why the analysis has to be run as a grid of state against product, not as a single question about the company.

Economic Nexus Is Not One Rule

Economic nexus — the principle that sufficient sales into a state can create a collection obligation without any physical presence — is now the general rule. But it is implemented separately by each state, and the differences are not cosmetic.

Four variables that differ:

The threshold amount. Not uniform.

What the threshold measures. Gross sales, retail sales, or taxable sales only — and whether exempt sales count. This is the single most common computational error: a wholesaler with substantial exempt sales that measures only taxable sales may be over a gross-sales threshold and not know it.

Whether transaction count is an alternative trigger. Some states include one, some removed theirs, and a low-price high-volume seller can cross a count threshold while nowhere near the dollar one.

The measurement period. Prior calendar year, current calendar year, or a rolling twelve months — which means the same sales history produces different answers, and a rolling measure requires monitoring rather than an annual review.

The consequence for practice: you cannot compute this once at the entity level. A client with identical sales in two states can be over in one and under in the other, and a threshold-monitoring process has to run against each state's own definition. Coverage sits in the broader sales and use tax training catalog.

Physical Presence Never Went Away

A point clients routinely misunderstand, having concluded that nexus is now purely about sales volume.

Physical presence still creates nexus, at levels far below any economic threshold. Common triggers:

Inventory held in a third-party fulfillment warehouse. The client may not know which states their inventory sits in — and the marketplace that placed it there is not the taxpayer for the client's own non-marketplace sales.

A remote employee. One person working from their home in a state can establish presence there, which is why the payroll question in our post on multi-state remote employees and the sales tax question arrive together and are usually handled apart.

Contractors or representatives performing services in the state.

Property, including equipment left with a customer.

Trade shows and in-person sales activity, which some states address with specific and narrow thresholds.

Installation, training, repair, or warranty work performed in-state.

The practical sequence: ask where the people, inventory, and property are before computing sales thresholds, because physical presence produces nexus at a lower bar and often at an earlier date.

Marketplace Sales: Two Traps

Marketplace facilitator laws shifted collection responsibility to the marketplace for sales made through it, which is genuinely simplifying. Two things it does not do:

It does not necessarily remove those sales from your threshold computation. States differ on whether marketplace-facilitated sales count toward the seller's own economic nexus threshold. Where they do, a business selling mostly through a marketplace can be pushed over a threshold by sales it never had to collect on — and then owes collection on its direct sales in that state.

It does not cover your other channels. A client selling through a marketplace and through their own site has two different answers in the same state.

Trailing Nexus

Under-discussed and occasionally decisive. Several states take the position that nexus continues for a period after the activity that created it stops — so closing a warehouse or losing a remote employee does not immediately end the filing obligation.

Which means the deregistration decision needs the same state-by-state check as the registration decision, and a client who stops filing the month after the activity ceases may be creating a delinquency.

The Liability Compounds Because Nobody Collected It

Worth stating plainly, because clients under-react until they understand this.

An unregistered seller's historical exposure is the tax that should have been collected from customers and was not. The business now owes it out of its own funds, with interest and potentially penalties, and it generally cannot go back and bill three years of customers for it.

Two implications:

The exposure grows with every month of non-compliance, and is not capped by anything.

It is a diligence finding. Undisclosed sales tax exposure is one of the classic items a buyer's quality-of-earnings work surfaces — see our post on M&A due diligence — and it is frequently what forces an escrow or a purchase price reduction. A client planning to sell has a stronger reason to fix this than a client who is not.

Voluntary Disclosure — Before Registration, Not After

The mechanism that exists for exactly this situation, and the reason the opening warning matters.

Most states operate a voluntary disclosure program under which a taxpayer that comes forward before being contacted can generally obtain a limited lookback period and penalty relief, often negotiated anonymously through a representative before the taxpayer is identified.

The eligibility conditions that CPAs must respect:

The taxpayer must not already be registered in many programs, and must not be under audit or already contacted about the tax.

Coming forward must be voluntary, which a response to a nexus questionnaire is not.

So the order of operations is: quantify the exposure, decide the strategy, then register — through whatever program applies. A registration filed first is often unrecoverable, and it is the specific error that turns a manageable cleanup into a full historical assessment.

Where exposure is small and recent, straightforward registration may be right. That is a decision made after the numbers exist, not before.

The Finding That Actually Comes Up in Audits

Not unregistered nexus. Exemption certificates.

A business making exempt sales bears the burden of supporting the exemption, and an auditor examining a registered seller routinely finds the same three problems: certificates missing for sales treated as exempt, certificates that are incomplete or expired, and certificates that do not match the transaction or the entity.

Without a valid certificate, the exempt sale is assessed as taxable — and the tax is again the seller's, not the customer's.

The controls that prevent it: collect the certificate before invoicing exempt, validate it on receipt, track expirations, re-solicit on a schedule, and keep them where they can be produced by transaction. The determination headaches session covers the taxability side.

Sourcing, Local Jurisdictions, and Use Tax

Sourcing determines which jurisdiction's rate applies. Most states source to destination for interstate sales, but intrastate rules vary and the answer for services and digital products often differs from tangible goods.

Local jurisdictions are where compliance actually gets hard. Some states administer local taxes centrally; others allow home-rule localities to administer their own, requiring separate registration, separate returns, and separate audits. A client "registered in the state" may still be non-compliant in cities within it.

Use tax is the mirror obligation and the one clients ignore entirely: the business owes use tax on its own purchases where the vendor did not charge sales tax — equipment, software, marketing materials, and anything bought from an out-of-state vendor who was not registered. Auditors examine purchases as routinely as sales, and a client with no use tax accrual has an exposure they have never thought about.

Taxability Areas Where States Diverge Most

Without asserting any state's current position — verify each:

Software and SaaS. Treated as tangible personal property, as a taxable service, as a non-taxable service, or differently depending on delivery method.

Digital products — downloads, streaming, e-books.

Services generally, where the default varies by state.

Bundled transactions, where a taxable component can taint the whole charge.

Shipping and handling, taxable in some states and not others, sometimes depending on how it is stated on the invoice.

Installation and labor, particularly on real property.

Income Tax Nexus Is a Separate Analysis

The question CPAs most often leave out of the engagement.

Sales tax nexus and income or franchise tax nexus are different determinations under different rules. Federal law has long protected a business whose only in-state activity is the solicitation of orders for tangible personal property, but that protection is narrow — it does not cover services, and states have taken increasingly assertive positions on which internet-based activities exceed mere solicitation.

Meanwhile, states apply economic factor-presence standards for income tax purposes independent of the sales tax thresholds.

The practical point for a multi-state client: one remote employee can create sales tax nexus, income tax nexus, and a payroll withholding obligation simultaneously, and those three answers come from three different bodies of law. The sales and use tax training courses and the multi-state payroll tax compliance program cover the two sides.

A Working Sequence

1. Map the footprint — people, inventory, property, contractors, and in-person activity, by state, with dates.
2. Pull sales by state for the relevant years, split by channel, with gross and exempt amounts separated.
3. Compare against each state's own threshold definition and measurement period.
4. Determine taxability of each product or service line in each state where nexus exists.
5. Quantify historical exposure by state and year, tax plus interest.
6. Choose a remediation route per state — voluntary disclosure, standard registration, or no action — before registering anywhere.
7. Fix prospective compliance: registrations, filing calendar, rate determination, exemption certificate process, and use tax accrual.
8. Institute threshold monitoring on a rolling basis, per state.

Where Businesses and Their CPAs Get Caught

• Registering before quantifying exposure, forfeiting voluntary disclosure eligibility
• Measuring taxable sales against a gross-sales threshold, understating the count
• Ignoring transaction-count triggers where a low-price seller crosses them
• Using one threshold and one measurement period for all states
• Assuming physical presence no longer matters, when it creates nexus far below any threshold
• Not knowing which states hold the client's fulfillment inventory
• Treating a remote employee as a payroll question only
• Excluding marketplace sales from the threshold where the state includes them
• Assuming the marketplace covers direct-channel sales
• Deregistering the month activity ceases, ignoring trailing nexus
• Answering a nexus questionnaire and then trying to come forward voluntarily
• Registering in the state but not in home-rule localities
• No exemption certificate process, which is the most common audit adjustment
• No use tax accrual on the client's own purchases
• Assuming SaaS is or is not taxable without checking the specific state
• Treating income tax nexus as answered because sales tax nexus was analyzed

The summary for a CPA with a multi-state client: run nexus and taxability as a grid rather than a single question, ask where the people and inventory are before computing any threshold, quantify the historical exposure state by state, and decide the remediation route before anyone registers — because registration is the one step that cannot be undone and it is usually the step a well-intentioned adviser takes first.

Frequently Asked Questions

What is the most expensive mistake in cleaning up unregistered nexus?

Registering before quantifying the exposure. Most state voluntary disclosure programs require that the taxpayer not already be registered and not have been contacted, so a registration filed first can forfeit the limited lookback and penalty relief the program offers — converting a negotiable historical liability into a dated admission.

Are economic nexus thresholds the same across states?

No, and four variables differ: the amount, what it measures (gross, retail, or taxable sales, and whether exempt sales count), whether transaction count is an alternative trigger, and the measurement period — prior year, current year, or rolling twelve months. Identical sales can be over the threshold in one state and under it in another.

Does physical presence still create nexus?

Yes, and at levels far below any economic threshold — inventory in a third-party fulfillment warehouse, a single remote employee, contractors performing services in-state, property left with a customer, trade show activity, and in-state installation or repair work. Ask where the people, inventory, and property are before computing sales thresholds.

Do marketplace sales count toward the seller's own threshold?

It depends on the state. Where they do, a business selling mostly through a marketplace can be pushed over a threshold by sales it never had to collect on, and then owes collection on its direct sales in that state. Marketplace facilitator collection also never covers the seller's other channels.

What do sales tax auditors most often assess?

Exemption certificates, not unregistered nexus. Missing, incomplete, expired, or mismatched certificates cause sales treated as exempt to be assessed as taxable — and the tax falls on the seller, who generally cannot recover it from the customer years later.

Is income tax nexus answered by the sales tax analysis?

No. They are separate determinations under different rules, and federal protection for solicitation of orders for tangible personal property is narrow — it does not cover services, and states have taken assertive positions on internet activities. One remote employee can create sales tax nexus, income tax nexus, and a payroll withholding obligation at once, under three different bodies of law.