Hello atltrng,
Thank you for participating in our Ask the Expert event today. As to your question, that depends. If you are vendor in the business of buying and selling coins then it will depend on the state, county, and even city you live in. If it was just a transaction between two private parties then you typically wouldn't have to pay any sales tax.
If your question was really about capital gains tax, you would only pay that on the profit you made.