In the case of interstate commerce (and catalog ordering, crossing the border to buy if you live nearby, etc) they have the concept of "use tax" where you must pay the equivalent of sales tax on goods you bought to use in your home state without paying state income tax. You are expected to pay use tax on goods you purchase on the internet tax-free at income tax filing time, and (a vanishingly small number of, I'm sure) honest people have been reporting their purchases and paying tax on goods purchased on the internet. Retroactively taking sales tax from those retailers would double-dip.

Sales taxes are imposed on the buyer, but is collected by the seller. That is why buyers are supposed to file use tax returns for out-of-state purchases.

This is in contrast to something like Hawaii's GET, which is a tax on the seller that is "passed on" to the buyer.