There is no one all-encompassing sales tax in the United States. The 50 individual states have their own tax laws and their own sales tax rates. These can differ immensely from state to state. In some states there is no such thing as a sales tax on precious metals, whereas in others you will pay close to an extra 10% on every purchase that you make.
It is therefore very important to understand the different laws and rates in your state. The same applies to individual counties, because even in a single state, there is very rarely just one rate that applies everywhere. Tax laws are the same state-wide, but the rates are not, and just by traveling out of the county you might be able to reduce the amount that you pay on precious metal purchases.