For the purposes of the OP (who is relatively new, I think), the terms "rare" and "scarce" have no accepted defined meaning in this hobby.
Hence we see many collectors complaining about sellers who use these terms...ahem...liberally.
There was an extensive dicussion of the term "rarity" in a thread on this board about a year ago. I think it would be tough to add a whole lot to that one:
http://www.net54baseball.com/showthr...ht=define+rare
Cheers,
Blair