Taxprof links to Travis Cavanaugh (J.D. 2012, Iowa), Note, Iowa Can do Better Than the Affiliate Tax, in which the author suggests how states could get to Amazon and make them withhold and pay over sales tax. I don't know why sales to customers can't be seen to generate the necessary nexus if the provision of a platform for resale could--probably I need to re-read International Shoe or something in addition to this Note. In international tax we learn that the U.S. does not typically consider sales alone to be sufficient to justify source-based taxation but this is mostly based on historical thought that nexus had to involve some sort of physical presence; exceptions have been made and someone could likely make the case that sales alone ought to suffice for international tax purposes. Mostly the impediment is not theoretical but practical, how to catch the thief.