Multi-State Sales Tax FAQ for Ecommerce Sellers

What is sales tax nexus?

Nexus is the legal connection between a seller and a state that triggers a sales tax registration obligation. Two main types:

• Physical nexus: Triggered by physical presence in a state. Includes warehouses, employees, contractors, and (importantly for ecommerce) inventory stored at FBA or 3PL locations.
• Economic nexus: Triggered by sales volume or transaction count into a state. Most states use a $100,000 annual sales threshold or 200 transactions, but the exact thresholds vary.

If you have either type of nexus in a state, you have a registration obligation regardless of the other type.

Do I need to register everywhere?

Almost certainly no. You need to register in every state where you have nexus AND a filing obligation. Marketplace facilitator laws mean that for many states, the marketplace handles the filing, and you don't need to register at all for those marketplace-facilitated sales.

However, if you have physical nexus in a state from inventory storage, several states still require you to register even if your only sales there are marketplace-facilitated. The rules vary widely.

If Amazon collects sales tax on my behalf, do I still file?

Maybe. It depends on the state. Three patterns:

• Don't file at all on marketplace-facilitated sales: California, Texas (with caveats), New York, and many others. Once Amazon remits, you have no further obligation for those sales.
• File an informational return showing marketplace sales but owing zero tax: A handful of states. You're not paying tax but you're acknowledging the sales activity.
• Still register due to physical nexus even though marketplace handles remittance: States with strict physical-nexus interpretation may want you on the registration rolls even if your filings are mostly zero.

For direct-checkout sales (your own Shopify checkout, your own site), the marketplace facilitator framework doesn't apply. You collect and you file.

How do I know where my FBA inventory is?

Amazon Seller Central, in the Inventory Reports section, has an Inventory Event Detail report that shows the fulfillment center locations holding your inventory. Pull this monthly. Your nexus map is built from this list.

ConnectStock surfaces the FBA state nexus automatically from the inventory data, alongside your 3PL state nexus and your home warehouse state nexus.

What's the back-tax risk if I'm not registered?

The risk depends on the state and on how long you've been operating without registration. State revenue departments do audit sellers for sales tax compliance, with marketplace data being one of their primary inputs (some states have data-sharing agreements with Amazon and other platforms).

If a state audits you and you've been holding inventory without being registered, they can assess back taxes (technically the customer paid via the marketplace, so for marketplace-facilitated sales the assessment is usually narrow), penalties, and interest. For sellers with direct-checkout sales into states where they weren't registered, the back-tax bill is larger because you collected from the customer and didn't remit.

States that have been most active on ecommerce seller audits: California, Texas, New York, Massachusetts, Illinois. Tax compliance specialists can run a voluntary disclosure agreement (VDA) to clean up prior years with reduced penalties.

What's a Voluntary Disclosure Agreement?

A VDA is an arrangement with a state revenue department where you proactively register, file back returns for a defined lookback period (usually 3-4 years), pay the back tax, and get reduced penalties. The state agrees not to look further back than the agreed lookback.

VDAs are a meaningfully better outcome than being discovered through an audit. If you have unregistered nexus in a few states, a sales tax specialist (TaxJar Plus, Avalara, or a sales tax CPA) can run VDAs in parallel for $300 to $1,500 per state in professional fees, plus the actual back tax.

Should I use TaxJar or Avalara?

Both are well-suited for ecommerce. TaxJar (now part of Stripe) is generally more affordable for sellers under $5M revenue and integrates well with Shopify and Amazon. Avalara is more powerful at the enterprise tier and integrates with more accounting platforms.

For most ConnectBooks customers under $5M, TaxJar is the right answer. Past $5M or with international operations, Avalara typically wins.

How does ConnectBooks fit into the sales tax workflow?

ConnectBooks handles the accounting side: separating marketplace-facilitated sales tax from self-collected sales tax, surfacing the inventory-driven nexus map, and producing the sales tax payable balance that needs to be remitted by jurisdiction.

ConnectBooks doesn't file your returns. That's TaxJar, Avalara, or your CPA. The handoff is clean: ConnectBooks tells you what's owed where; the filing service takes it from there.

What's the biggest mistake sellers make?

Operating for years with inventory in FBA states without registering. This creates a back-tax exposure that grows quietly and gets expensive when discovered. The fix is registration + VDA, but it's a several-month project and a real out-of-pocket cost.

The second-biggest mistake: collecting sales tax on direct Shopify sales into states where you're registered, then never remitting. The state collects from the customer and never sees the money. This is much worse than not collecting at all.

Am I going to get audited?

For a $1M-$5M revenue ecommerce seller, the audit probability in any given year is low (single-digit percent), but the audit cost when it happens is meaningful (5-15K in professional fees plus assessed tax). The math favors getting registered correctly now rather than waiting for the audit.

For sellers over $10M revenue, the audit probability rises substantially. The states have data and they use it. Compliance discipline at that revenue tier is non-negotiable.

I just hit the economic nexus threshold in a new state. How fast do I need to register?

Most states require registration within 30 days of crossing the threshold, with the first filing due in the next regular filing period for that state. Don't sit on it; it's straightforward and the registration is usually free or low-cost.

Can I voluntarily collect sales tax even where I don't have nexus?

Generally yes for direct-checkout sales, but it's almost always a bad idea. You take on filing obligations without a clear benefit, and any over-collection has to be remitted or returned to the customer.

What's the safest reading of multi-state sales tax compliance?

Get registered everywhere you have physical nexus (driven by inventory location), get registered everywhere you've crossed economic nexus thresholds, let marketplace facilitators handle the marketplace tax, and file your direct-checkout sales tax in your registered states. ConnectBooks gives you the data; a specialist handles the filings.

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