Do I pay tax on cash from coins I sold?

not an easy question to answer 

Whether a hobbyist or collector, the coins in your collection are considered a capital asset under the income tax laws. Accordingly, gains from the sale of a capital asset are either considered short-term or long-term. Long-term is when you own the coin sold for greater than one year. The long-term capital gain tax rate for the sale of collectibles (i.e. coins) is capped at 28%. If you are an investor and sell any coin at a loss, then the loss is deductible short-term or long-term depending on the holding period

But what if it's a hobby. then gains are taxable and losses are not deductible. if you are collecting coins solely for personal reasons, it’s a hobby.

If you are buying coins purely for pleasure and do not seek a profit, then it’s clearly a hobby. If you are buying coins solely to seek profits, then you are an investor. Typically, most collectors collect coins for a combination of reasons, including both - pleasure and profit. In these cases, the income tax laws aren’t so clear as to whether you are a hobbyist or an investor.