While I agree the ebay seller "may not keep track of who he sold it to", he may keep track. Also, some of the auctions show the name of the buyer. Even if the auction doesn't show the buyer's name, many times you can figure out who bought it by looking at feedback. So, that would be some possible leads, which beats the heck out of no leads at all. If I was looking for a long lost part, I would be thrilled to have an ebay lead, even if it were a few years old.
Just my 02 cents.