Are you buyer or seller, you refer to SALE to USA , then mention BUYING from USA.
If you buy from outside the UK or seller is registered outside the UK, then you are IMPORTING and pay VAT on items less thsn £135, above that figure Import Duty is charged and the company delivering has to collect this and charge about £14 to collect the duty. (that is I believe what Royal Mail charge, Couriers charge more).
If you sell to the USA, you pay no tax, but individual States have their own tax on purchases, which ebay include on invoices, and Collect for the US States.
I see you are registered in the USA, so you should ask in the USA, as my answer above is based on what happens for buyers and sellers in the UK.