I to have wondered about the term "rare" on ebay. I see this term used alot and usually by people I have never heard from. Many times they mistake "Limited Edition" to stand for rare. Non-chippers bring home a chip with "1 of 2500" on them and assume it must be rare.
I consider rare to be chips that are hard to obtain. There might be 100 of these chips out there, but if they are controlled by one person who is not giving them up, this could constitute rare. I know of people sitting on boxes of chips just so their one in their collection could be considered rare. To me, it depends on availability as much as population.
I don't like the word rare used on ebay. I listed one as "moderately rare" because I feel it is difficult to obtain, but not impossible.
Hope it helps,