Do you mean sales tax, or tax on profits? There isn't a Federal Sales Tax... yet. Utah doesn't tax coins or currency because then you would have to pay tax at the bank if you wrote a check for cash.
If you sell a coin and make a profit, and report it, it's the same as income as anything else and would be taxed at whatever rate/ bracket you are in. Ask a tax preparer or a CPA or try Google. Unless the value is very high and it's not a cash deal, there's not much of a paper trail. (If you buy and sell in cash.)
For federal purposes, when coins are bought and sold by private individuals any proceeds are treated as capital gains and taxed at that rate. And it doesn't make any difference what your income is, capital gains have their own specific rate. Not sure what it is today but the last time I had to worry about it it was 28% for short term, and 20% for long term. Regarding the question of buying a lot and selling individual coins, in that case the proper value percentage of each coin must be assigned and used as its basis, then capital gains rates apply. What a lot of folks never seem to realize or even think of is that losses on coins can also be utilized to offset gains on coins in equal amounts. And unused losses can be accumulated for 3 years. For this reason, as well as all the usual reasons, it is usually best to have an accountant do your taxes because they know all the ins and outs far better than most people. Lastly, certain individuals have different situations and so things may vary. But only an accountant can tell you if they do or not - so use one !
Capital gains only kicks in if they have been held for over a year. For less than a year it's treated as regular income and taxed at that rate. Coins now only have one capital gains rate, 28%.
