Cards some collectors think are rare, but are not.

[Cory]

And I think the real issue is rarity vs. scarcity as mentioned above. Just because something is rare doesn't make it valuable. And just because something isn't "rare" in you definition doesn't mean it can't be valuable if there is more demand than supply. It all boils down to supply and demand.

t206's are the perfect ecosystem to demonstrate this -

The Armbruster PB mentioned above is presumably just as rare as the Demmitt STL. But most t206 collectors chasing the monster don't care if Armbruster has a Piedmont or Sweet Cap or Polar Bear back as long as they have one, which broadens the supply. However, everyone that wants to complete the 520 needs a Demmitt (and O'Hara) STL which was only printed with a PB back. There are only 200 in the PSA pop, so if this is half the available population, say 400 exist. If there are 1000 people chasing the monster there is more demand than supply so the price goes up- so the Armbruster is just as "rare" as the Demmitt STL, but not nearly as scarce.