The laws changed after South Dakota v Wayfair. Each state has it's own nexus which can be by volume of sales, dollar amount or both. That is assuming you(the business) has no property or w4/9 "empliyees" living and working within the state being sold in to.

I'm surprised by how few people actually seem to understand the fees associated with each of the various selling options.

I constantly hear people comparing PWCC or Probstein fees to eBay even for higher end cards, saying something like "eBay charges 12.5%", so you're better off selling through Probstein anyhow because they only charge 10% or some such nonsense. If you have an ebay store (which only costs something like $20 per month), then those 12.5% selling fees only apply to the first $2500. Any amount above that only gets charged a 2.5% payment processing fee. If you sell a card for $15,000 on ebay, your total fees will be $625 (12.5% of $2500 + 2.5% of $12,500), which is only 4.2%. If you sell that same card through Probstein for $15,000, you'll pay $1500 in fees since that tier comes with a 10% consignment fee. And even if you sell through PWCC with their 20% buyers premium model, you're still not paying anywhere near 20% in actual fees as they give you a percentage of that buyers premium. For a card that sells for $15,000, they give you 110% of the hammer price, or half of the buyers premium. But even that still doesn't work out to paying a 10% fee. To figure out the true percentage, you take the amount you receive divided by the amount the buyer actually pays. So if they paid $15,000 with the buyers premium, then the hammer price was $12,500 + a $2500 buyer's premium, which you get half of. So, you would pay $1250 out of the $15,000 in fees to PWCC, which is 8.3%.

Cliff notes - if you sell a $15,000 card your fees would be
$625 on ebay (4.2%)
~$675 on myslabs.com (~4.5%, NOT 1% as advertised)
$1250 on PWCC (8.3%)
$1500 through Probstein (10%)

And even for the cheapest cards on PWCC that only sell for less than $50, which comes with the full 20% buyers premium going to PWCC, you still aren't paying them 20%. You're paying $2 in fees for a $12 sale, which is 16.7%, not 20%.

In rebuttal to what some have claimed above, buyers always know exactly how much they're paying on PWCC. The buyers premium is not hidden. It's not up to them to figure it out on their own. When you go to place a bid, it states right there how much your total bid is including the buyers premium. So they're not trying to trick people into thinking they are paying less and suckering them in for another 20% at checkout as implied above. It's expressly stated right there on your bid amount.

Make more, work less. Sounds like a winner to me.

Disclosure: not a fan or PWCC buyer since 2016.