Sort of related...
Ive picked up a card for $5 on eBay that Id never seen, then sold it for 100X more once 6 more from same series was discovered. (I call em ca1923 Sepia "Shoulderless" cards), they are still uncataloged years later, and the same 7 are the only known cards.
Right now on eBay I have a card bookmarked that is uncataloged, but from a very rare cataloged set. commons sell for $300+ in EX, yet this card should sell for less than 10% of that because the seller didnt ID or list it correctly.
(more info once auction is over
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