Hey I see people in the paper trying to sell old used PC's worth maybe $100 for $500, they wont sell. Alot of the mint unused inbox A1200's that are listed for $500 buy it now never sell, or sell to 0 feedback people who never pay. The more you see "RARE" in a description the more you should stay away from the auction. And I do see alot of fake/ripoff items there sold as prototypes that somebody ends up paying alot for (fake apple IIgs in apple II boxes etc).
The thing with EBAY is one persons junk is another persons hobby.. if you list junk long enough somebody WILL buy it. You do need to do some research before you bid on the first thing in front of you.
I always check what shipping/handling is before I even read the full details.. I wont bid on anything if they gouge on handling. Back when I was looking for an old 386 retro gaming rig I noticed alot of people selling junk pc's with opening bids for $1 but jacking the handling charges up like crazy, I wonder how many people purchased an item without looking at the hidden charges.
Rare could also mean rare in your area, it takes alot of money/time to run around trying to find a missing game/part for your retro system and ebay fits the bill for this.
Check out the non forsale product specific newsgroups, people sell stuff you need for next to nothing (or just for postage) just because your in the same hobby they are.