Does rarity help or hurt value?

[Topnotchsy]

An important point that has been touched on to a degree by a few posters (but may not have been fully explicated) is the way that with collectibles, supply (and other factors that don't necessarily play a role with traditional items) can play a huge role in demand.

Set collectors (for example) will rarely chase a set that is basically impossible to achieve. The size of the group of collectors chasing the set is likely constrained by rarity. If there are only 2 copies in existence of each card, having more than a few collectors chasing it would be very tiring indeed, and like lead to some collectors dropping out. Sets like T206 (outside of the big 4) and 1933 Goudey have enough copies for hundreds (thousands?) of people to collect the set, while still being somewhat rare.