[mikeash]

Hasn't this long been answered with "yes" for #2 and "no" for the others? When I buy from Amazon, they pay sales tax where I live, but not where the item came from or any other place where they have a presence.

The only wrinkle is that #2 is not very well enforced when a company doesn't have a presence in the buyer's state, but it's still technically required.

[ensignavenger]

No, if the buyer and seller do not reside in the same state (an by reside, for the seller, it includes having a business "nexus") then it is interstate commerce, and states can't require sellers to collect sales tax. (They usually instead impose use tax on the buyer).

[mikeash]

I don't understand, what's "no" about that? I didn't say sellers were required to pay tax, merely that paying tax was still required. As you say, it becomes the buyer's responsibility if the seller doesn't have a presence in the state.