What it is

South Dakota v. Wayfair, Inc. is the U.S. Supreme Court decision, handed down on June 21, 2018, that rewrote the rules for when a state can make an out-of-state business collect its sales tax. By a 5-4 vote, the Court overruled its own 1992 decision in Quill Corp. v. North Dakota — and the older National Bellas Hess case — holding that the long-standing “physical presence” requirement was “unsound and incorrect.” <!– src: https://www.supremecourt.gov/opinions/17pdf/17-494_j4el.pdf –><!– src: https://www.salestaxinstitute.com/resources/u-s-supreme-court-issues-decision-in-south-dakota-v-wayfair –>

Before Wayfair, Quill meant a state could only force a seller to collect sales tax if that seller had a tangible footprint there — an office, employees, or inventory. A purely out-of-state mail-order or online seller could ship into a state and owe nothing. Wayfair ended that. The Court ruled that a seller “need not have a physical presence in a State” to be taxed, and that the real test is whether the seller’s activity has substantial nexus with the state. <!– src: https://www.salestaxinstitute.com/resources/u-s-supreme-court-issues-decision-in-south-dakota-v-wayfair –>

The specific law in front of the Court was South Dakota’s. It required any business with more than $100,000 in sales or 200 or more separate transactions into the state to collect and remit sales tax, even with no property or staff there. The Court upheld that law, and in doing so created the template for the economic nexus rules that now exist in every sales-tax state. <!– src: https://en.wikipedia.org/wiki/South_Dakota_v._Wayfair,_Inc. –>

Why it matters to a multi-state seller

Wayfair is the single most important reason a small e-commerce or wholesale business can suddenly owe sales tax in dozens of states without ever leaving home. The decision didn’t itself impose tax everywhere — it permitted states to do so. Within roughly two years, almost every state that levies a sales tax passed its own economic-nexus law modeled on South Dakota’s.

For you, that means the question “Do I have a warehouse there?” is no longer the right question. The right question is “Did my sales into that state cross its threshold?” A seller shipping nationwide from a single location can trip nexus in 20+ states purely on revenue. The obligation — registering, collecting the right rate, and filing returns — follows the sales, not the building.

The flip side is that Wayfair also blessed thresholds. South Dakota’s law specifically protected small sellers below $100,000 or 200 transactions, and the Court pointed to those safe harbors as part of why the law was constitutional. So Wayfair both expanded your potential exposure and gave you a floor below which a given state can’t reach you.

Worked example

Suppose your online brand ships from one location in Florida and, over the last 12 months, recorded these out-of-state sales:

• South Dakota: $90,000 across 210 transactions. Under the law Wayfair upheld, the test is “$100,000 or 200 transactions.” You’re under the dollar figure but over the transaction count → nexus is triggered. You must register and collect. <!– src: https://en.wikipedia.org/wiki/South_Dakota_v._Wayfair,_Inc. –>
• A neighboring state with a $100,000 sales-only threshold: $90,000 in sales, 210 orders → under $100,000, no transaction test → no nexus yet.

Same order volume, two different outcomes — entirely because Wayfair let each state set its own threshold, and not all of them copied the transaction count. This is why post-Wayfair compliance is fundamentally a state-by-state monitoring problem.

How this connects to staying compliant

Wayfair is the why behind almost everything in modern sales-tax compliance. Because of it, you need to: (1) track rolling sales by state, (2) compare them to each state’s current economic-nexus threshold, (3) register once you cross, and (4) collect and file going forward. Ignoring a state where you’ve crossed the line doesn’t make the liability disappear — the state can assess back tax, penalties, and interest, and that bill is yours, not your customers’.

The practical takeaway: Wayfair turned sales tax from a “where am I physically located” question into a “where do my customers buy” question. The defense is continuous monitoring rather than a one-time setup.

Sources: South Dakota v. Wayfair, Inc., 138 S. Ct. 2080 (2018) — full opinion: https://www.supremecourt.gov/opinions/17pdf/17-494_j4el.pdf Sales Tax Institute — U.S. Supreme Court Issues Decision in South Dakota v. Wayfair: https://www.salestaxinstitute.com/resources/u-s-supreme-court-issues-decision-in-south-dakota-v-wayfair South Dakota v. Wayfair, Inc. — case summary (date, vote, thresholds, overturned cases): https://en.wikipedia.org/wiki/South_Dakota_v._Wayfair,_Inc.