what did you pay for them? don't answer that obviously, but that's your start off point. you don't pay any tax on the non profit. if that $6000 worth of coins cost $3000 to buy then you pay 10% of $3000.

there's usually tax free allowances so if you only sell so much in a tax year and it's below the non taxed threshold then there won't be any tax to pay. i don't know what the deal is in america but that's how it is elsewhere. it sounds like you should get the opinion of a pro.