Posted By:
HankronExluding the eBay sellers who use terms 'rare', 'very rare' and 'original' simply as devices to gather a few extra bucks on their auctions, I've found that the baseball card hobby uses the terms 'rare' and 'scarce' largely accurately.
Looking at the hobby, dealers/collectors call a card 'rare' if there are strickly very few in existence (the Honus Wagner is rare). While 'scarce' is used to label a card that is too plentiful to be called rare, but is limited in number and difficult to obtain if you want one. Certainly the 1974 Topps 'Washington Nationals' variations are not rare. But, if you are trying to finish the set, you know these much tougher to get than the other cards.
Another common term for scarce that I like is 'tough.' Meaning, irrelevant to how many are theoritically out there, if you want one it's tough to get. Often relates to condition. "The 1978 Topps Molitor/Trammell (I was born in Wisconsin, so Molitor goes first) are easy to find, but tough to get in high grade because of the print smudge."