I got audited a few years ago, maybe this will help somebody. I had bought a couple of watch from overseas, one person-to-person deal, another from a dealer's own website. Since they were international, I expected to pay import customs duties (this was before the additional Trump tariffs). And of course, no sales tax. They got held up on arrival at customs clearance, as usual. The P2P one took a week, waiting on paperwork from the seller to calculate customs before UPS invoiced me to release it. Dealer one was experienced and got released quicker, DHL paid the customs on my behalf and invoiced me after delivery for the reimbursement plus service fee. 2 years later, I get a notice from the STATE Treasury Department at the local level, not the IRS at the federal level, saying I owe a ton of back taxes on the watches.

Sales tax applies on everything you buy for your own use. Most of the time, the store collects it and pays the state government. And if it wasn't collected (out-of-state and international purchases), the buyer is still technically responsible for it on like an honor system, some states call it a Use or Consumption or Stamp Tax, that you're supposed to file per year of import (could be a line item in your regular taxes, or a separate form for later filing). Uncollected tax is kind of "don't ask, don't tell". Except these international purchases had a paper trail because of the customs import process, documenting the value of what I bought. Dunno if I just go unlucky with the spot-check lottery, or if the Feds are narcing on everyone to the states now. (though something similar happen a decade earlier to a friend importing handbags. some "pre-loved" ones got confiscated/destroyed for lack of CITES tags, others passed thru after paying customs, paper trail lead to sales tax scenario like mine. only difference was her packages were wrapped tight like bricks of powder and got pulled for inspection, initially made me think it only happened to her because she got busted at something, always assumed I'd be ok since mine are documented and not deliberately dodging anything)

So I owed the unpaid sales tax, the interest on all the unpaid days, and a penalty. The penalty is meant to be punitive to really discourage noncompliance, so the total owed was almost double the amount of simply the unpaid tax. I pled ignorance, first offence, etc and negotiated most of the penalty portion away. But the negotiated settlement also has a deadline, missing it means the interest continues to accumulate.

International deals will have different combinations of collected/uncollected State sales/use tax, Federal customs, Federal Trump tariffs. The big platforms like C24 and Ebay already collect tax (and I've seen some ebay listings that will add in customs and/or tariffs too, but not sure I trust their calculations of item origin vs seller origin). But, for example, the folks that won M.A.D. raffles might have exposed liability. Since I already used my Plea Ignorance card, I need to be more mindful in the future.

I wonder if buying something DDP ('Deliver Duties Paid' by seller already, instead of the usual DAP 'Deliver At Place' and buyer pays duties), which may skip the import delay, would mean less or different paper trail. Or just buy domestic, hellooo sweet marketplace!