That is categorized as a collectible. In order to determine the gain on the sale, you will want to find out what the person paid for the coin who gifted it to your husband. If it was purchased as a collectible, what your husband can claim is the original purchase price or the fair market value on that day it was obtained. (Since it was a $1 coin, the original value was at least $1, but could have been more).
Without that information, you pay tax on virtually the entire sales price. And since it is a collectible, the tax rate can be higher than typical capital gains tax rate: your marginal tax bracket rate, or 28%, whichever rate is lower.
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