Not specifically rare coins, but when I was cashing out bitcoin for silver, I tried to collect "weird" stuff from new or less common mints.
I've got a lot of goofy 1oz silver rounds. The premium was maybe $0.50 over the bland, boring stuff (buffalo, Apmex, etc). I've sold a few (bitcoin -> silver -> eBay -> PayPal -> bank account), and the weird stuff seems to go for a premium. At the worst, it's still an ounce of silver.