[jasode]

>The theory is that they are the actual seller.

Right, South Carolina in particular wants to change the "seller of record" from the marketplace sellers to Amazon itself. Most of the other states are targeting the marketplace sellers and they want more of Amazon's help in that. (E.g. add explicity sales tax line items like Ebay and provide audit reports to help identify sales tax evaders.)

[ghaff]

Which, if sellers are going to collect sales tax at all, is probably the only thing that makes sense. It's impractical for small sellers to file tax returns for all the states they sell to just because they sell on Amazon. (And in the vast majority of cases, non-Amazon sellers without a presence in the buyers state don't and, indeed, can't be forced to.)