Physical coins? In crypto market, you have digitalized coins or tokens, not physical.
Depends on your nation, you have to obey with your national rule on tax. Generally, if you end with profit, you need to pay tax for governments.
[Tutorial] Crypto taxes for beginnersI think what OP meant was those ancient coins (tens or hundred years old kind). Collectors item.
If it's BTC, most likely it's gonna be taxed since you have to declare the profits or losses as cryptos are treated as personal assets:
Cryptocurrencies are treated as personal property (not currency) and are therefore taxed as capital assets.
Capital gains from selling cryptocurrency for fiat currency (e.g. USD) or using cryptocurrency to purchase goods or services are subject to capital gains tax
Source:
https://www.irs.gov/pub/irs-drop/n-14-21.pdf