Sales tax in the US is charged at the state and local level only. There is no national VAT or GST like there is in Europe. The USPS operates as a quasi-federal goverment agency, and does not get involved in local matters like sales tax. So that is never an issue here.

Technically, a resident of a state is supposed to pay a tax on items brought into the state, if the state has a sales tax. There's only 3 or 4 states that don't have one. However, as you can imagine, these laws are almost impossible to enforce, particularly since the post office will not collect the tax. eBay will collect and pay sales tax if the mailing addresses of buyer and seller are in the same state. Most live auction houses will also collect it, unless the buyer doesn't retrieve the item in person and it is shipped out-of-state.

So the issue of sales tax is almost as complicated as the federal customs duties.

Some states have taken to putting a nominal charge, usually between $50 and $100, on their annual state income tax returns for this "use tax" and letting residents voluntarily adjust it up or down (under threat of audit).

If asked, I always advise clients to pay it and be quiet.
gatorcpa