Sooooo..... if California, or any state, wants to collect this tax, charge the seller for what is sold in said state. Too problematic, you say? Too bad. I have no sympathy for the tax collector.
Anyway, even if the tax is supposed to be on the one doing the buying (which I disagree with), maybe it should be changed and make the seller responsible. After all, he's the one earning something. I think tax should be on the profit earned.
There are two types of taxes here: sales tax and use tax.
Generally, a seller is required to collect the sales tax from the buyer and then remits that to the state. The seller can pass on collecting the tax but is still required to remit the amount to the state.
One of the exceptions is in interstate commerce. The seller is not required to collect sales tax in interstate commerce.
In the case of interstate commerce, the buyer is required to remit the tax to the state. In this case, it is called a use tax.