I've been watching and bidding on some ebay LEGO auctions by Richard Morton.  Apparently Richard used to be really into LEGO.  He has TONS of rare pieces in new condition.  Several of his auctions are for pieces that have never been in a set.

For example he has a bag of 200 clear 1X2 plates with 1 stud up for action right now. There are 9 people on Bricklink who are selling this piece.  One seller even has 932 of the part...for 20¢ a piece.  You can buy the piece in clear if you want, but it's gonna cost ya.

I certainly don't have any clear 1X2 plates with 1 stud, and I don't recall ever seeing one in real life.  When I came across that ebay auction, my jaw dropped.  "Clear 1X2 plates with 1 stud!" I thought to myself.  They're ultrarare, and the guy has a whole bag of them.  Then I realized that although their resell value would be good (assuming that the bidding didn't go up to $40), I wouldn't have any immediate use for that piece in clear.  Clear 1X2 tiles yes, but 1X2 plates with 1 stud, no.

I thought my reaction to seeing that bag of pieces was sort of funny.  It wasn't a piece that I was looking for, but it quickly grabbed my attention just because it was a lot of a rare part.

UPDATE: The auction for the clear 1X2 plates with 2 stud ended at $15.52.