Rarity is far from the only determiner for price. In fact, some cards are too rare for their own good: obscure.
I knew of a person who had a cloth tintype: a tintype on cloth instead of metal and mounted in form of a CDV. I knew they were possible but that was the first and only example I have ever seen. They guy had troubles getting a fair price, as almost no one knew or understood what it was.
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