"The taxes that the buyer will pay will be based on the final/agreed upon price for the item + shipping."
The best offer process refers only to the item's price. The postage/shipping cost will remain the same. After all, the item's size and weight don't change, and those are two factors involved in finding the correct postage.
As for sales tax, that depends on what state the item will be shipped to.
Some states don't charge sales tax at all.
Some states do charge tax on the postage/shipping cost, if it is listed separately.
If the buyer's state does not tax postage, then the tax would be on only the accepted price of the item.
If the item is sold on eBay with "free shipping", the item's price should include postage, or at least part of the postage. In that case, the buyer (in a state that does tax) essentially does pay tax on "postage."
