Sales Tax Nexus Explained: What US E-commerce Sellers Need to Know

If you sell online in the US, you’ve probably heard the term “nexus” thrown around — usually right before someone mentions an unexpected tax bill. Sales tax nexus is one of the most misunderstood parts of running an e-commerce business, and getting it wrong can mean owing back taxes, penalties, and interest in states you didn’t even realize you had obligations in.

This isn’t a niche concern either — as e-commerce has grown, so has state enforcement. Many states now actively cross-reference data from payment processors, marketplaces, and fulfillment networks to identify sellers who should be registered but aren’t. Here’s what nexus actually means, how it applies to online sellers, what it costs to get wrong, and exactly what to do about it before it becomes a problem.

What Is Sales Tax Nexus?

Nexus is the legal term for a connection between your business and a state significant enough that the state can require you to collect and remit sales tax there. If you have nexus in a state, you’re generally required to register, collect sales tax from customers in that state, and file returns — regardless of where your business is physically based.

Before 2018, nexus was mostly tied to physical presence — an office, warehouse, or employee in a state. That changed with the Supreme Court’s South Dakota v. Wayfair decision, which opened the door for states to require tax collection based on sales activity alone, even without a physical presence. Within a couple of years, nearly every state with a sales tax had adopted some form of economic nexus law, fundamentally changing what “doing business” in a state means for online sellers.

The Two Types of Nexus E-commerce Sellers Need to Know

1. Physical Nexus

This is triggered by having a physical connection to a state, including:

• An office, warehouse, or storefront
• Employees or contractors working in the state
• Inventory stored in the state — this one catches a lot of sellers off guard, especially those using fulfillment networks that distribute inventory across multiple warehouses automatically
• Attending trade shows or markets in a state, in some jurisdictions, if it happens with enough regularity
• Third-party affiliates or referral partners based in a state, under certain state “click-through nexus” rules

2. Economic Nexus

This is triggered purely by sales activity, regardless of physical presence. Most states set a threshold, commonly structured around:

• A dollar amount of sales into the state within a 12-month period (often $100,000, though this varies by state)
• Or a minimum number of separate transactions into the state (some states also use a transaction count threshold, such as 200 transactions)

Once you cross a state’s threshold, you’re generally required to register and start collecting tax there — even if you have no physical presence at all. It’s worth noting that thresholds are not static; several states have adjusted or removed transaction-count thresholds in recent years, so a rule that applied when you last checked may no longer be accurate.

Why This Catches So Many E-commerce Sellers Off Guard

The core problem is scale and visibility. A seller might not realize they’ve crossed an economic nexus threshold in a state until months later, since most sales platforms don’t proactively flag this for you. And because thresholds and rules vary state by state, a business selling nationally can end up with nexus obligations in a dozen or more states without ever making a conscious decision to “expand” there.

Fulfillment-based nexus adds another layer — if your inventory sits in third-party warehouses spread across different states, you may have physical nexus in states you’ve never actually operated in yourself. Sellers using large third-party fulfillment networks are especially exposed here, since inventory placement decisions are often made algorithmically, not by the seller, yet the nexus consequences still fall on the business.

Growth itself is often the trigger. A seller who was well under every threshold two years ago can quietly cross several state lines’ worth of obligations after a strong sales year — without any change in how or where they operate.

What Happens If You Don’t Comply

States that determine you had nexus and failed to collect tax can pursue back taxes, penalties, and interest — sometimes going back several years, depending on the state’s statute of limitations and whether a return was ever filed. Unlike income tax, sales tax is money you were supposed to collect from customers, not pay out of your own margin, which makes an unexpected assessment especially painful since that money was never set aside.

In more serious cases, prolonged non-compliance can also affect a business’s ability to get financing, pass due diligence during an acquisition, or maintain marketplace selling privileges, since buyers and platforms increasingly ask about sales tax compliance history.

Marketplace Facilitator Laws: What Changed

Most states have now passed “marketplace facilitator” laws, which shift the responsibility for collecting and remitting sales tax to platforms like Amazon, Etsy, or Walmart Marketplace for sales made through their platform. This has simplified compliance for many sellers, but it comes with an important caveat: marketplace facilitator laws typically only cover sales made through that marketplace.

If you also sell through your own website, a different marketplace not covered by facilitator rules, or wholesale channels, those sales are usually still your direct responsibility. Many sellers mistakenly assume that because Amazon “handles their taxes,” they’re fully covered — only to discover a gap once they add a Shopify store or start selling wholesale.

How to Get Ahead of Sales Tax Nexus

1. Run a Nexus Study

A nexus study reviews your sales history — by state, by channel, by transaction volume — against every state’s thresholds to identify exactly where you currently have, or are approaching, an obligation. This is the starting point for any serious compliance effort, since you can’t fix what you haven’t identified. A proper study also accounts for physical nexus triggers like inventory location, not just economic thresholds.

2. Register in States Where You Have Nexus

Once identified, registration needs to happen before or as close as possible to when the obligation started, to minimize the exposure window. Registering too early, in a state you don’t actually have nexus in yet, can also create unnecessary filing obligations, so accuracy matters in both directions.

3. Consider a Voluntary Disclosure Agreement (VDA) If You’re Already Behind

If a nexus study reveals you should have been registered in a state for some time already, many states offer Voluntary Disclosure Agreements — a formal process where a business proactively comes forward before being audited, typically in exchange for a limited lookback period and reduced or waived penalties. This is almost always a better outcome than waiting for a state to identify the gap on its own.

4. Set Up Automated Tax Collection

Manually tracking tax rates across states isn’t realistic at any real sales volume — rates and rules change too often, and many states have different rates by county or even city. Automated tax collection tools integrated with your sales platform calculate the correct rate in real time at checkout, based on the customer’s location.

5. File Consistently, Even in Zero-Sales Months

Registration without consistent filing creates its own problems — most states expect regular returns even in months with no sales in that state, and missed filings can trigger penalties on their own, separate from any tax actually owed.

6. Revisit Your Nexus Status Regularly

Nexus isn’t a one-time determination. As your sales grow, shift between channels, or expand into new states, your nexus footprint changes with it. A study done once during a slow growth period can become significantly outdated within a year or two of strong sales.

A Realistic Example

Consider a seller doing $40,000/month in sales, split roughly evenly across five states, with inventory stored in a fulfillment network that distributes stock across additional warehouses in three more states. Within a single year, this seller could plausibly cross economic nexus thresholds in several of their top-selling states, while also holding physical nexus in the states where their inventory happens to be warehoused — resulting in filing obligations in eight or more states, despite the business itself operating from just one location.

This scenario is common, not unusual, and it illustrates why nexus needs to be actively monitored rather than assumed to be “handled” once initial registrations are done.

How This Connects to Your Broader Bookkeeping

Sales tax compliance doesn’t exist separately from your financial records — accurate bookkeeping and accounting is what makes a nexus study possible in the first place, since you need clean, accurate sales-by-state data to know where you actually stand. Businesses with disorganized books often discover nexus issues later than they should, simply because the data needed to catch it wasn’t being tracked properly.

For ongoing compliance once you’re registered across multiple states, ongoing tax filing and compliance support ensures filings happen consistently and on time, rather than becoming another recurring task competing for your attention.

Common Questions About Sales Tax Nexus

Does this apply if I only sell through a marketplace like Amazon or Etsy?
Many states have marketplace facilitator laws that shift collection responsibility to the platform itself for marketplace sales. However, if you also sell through your own website or other channels, those sales may still create nexus obligations you’re responsible for directly.

How often do I need to check my nexus status?
Ideally on an ongoing basis, since thresholds are typically measured on a rolling or annual basis — a nexus study done once and never revisited can quickly become outdated as your sales grow or shift across states.

What if I’ve already crossed a threshold and didn’t know?
This is more common than most sellers realize, and it’s better addressed proactively than left until a state notices. Voluntary disclosure programs in many states can reduce penalties for sellers who come forward before being audited.

Does nexus apply to international sellers shipping into the US?
Yes — economic nexus thresholds generally apply regardless of where the seller is based. A non-US business selling into American customers can still trigger nexus obligations the same way a domestic seller would.

Can I just collect tax everywhere to be safe?
Collecting tax in a state where you don’t have nexus isn’t a safe default — it can create its own compliance and refund complications, and in some cases isn’t legally appropriate. The right approach is accurate registration based on where nexus actually exists, not blanket collection.

A Quick Nexus Health-Check

Ask yourself these questions to get a rough sense of your exposure:

• Has your sales volume grown significantly in the past 12 months, in any state?
• Do you use a fulfillment network that stores inventory across multiple states?
• Do you sell through more than one channel (your own site plus a marketplace, for example)?
• Has it been more than a year since you last reviewed your nexus footprint?

If you answered yes to more than one of these, it’s worth having your nexus status formally reviewed rather than assumed.

Getting This Right Before It Becomes a Problem

Sales tax nexus isn’t something that resolves itself, and the longer it goes unaddressed, the more exposure builds up. A proper review of your sales data against current state thresholds is the only reliable way to know where you actually stand — and the earlier it’s done, the more options you have to fix any gaps on favorable terms.

We help US e-commerce businesses identify nexus exposure, register correctly, and stay compliant across every state they sell into. Book a free consultation and we’ll walk through exactly where your business stands.