eBay does have a real mix of false advertising and simply ignorant advertising, but in my experience there are a lot of honest sellers who don't know quite what they have but are willing to answer questions asked early in the auction. I'd say my best buys on eBay have been photo equipment, not bike stuff, and when I've gotten great deals, it's always been a matter of figuring out things that weren't stated in the ad.
My single best buy to date was an auction in the scuba gear section for an Ikelite case to fit Olympus OM-1 cameras, with accessories. The heading only mentioned the "case, camera, and accessories", and the seller was a diver but not a photographer so the ad didn't have much detail on what was included on the photographic side. I enlarged the ad photos and sharpened them up, then asked questions to confirm what I saw: the auction included an Olympus OM-2N with Zuiko 24mm f/2 lens, and also included a 400WS Ikelite strobe. I wrote back to the seller to let him know what the various parts I'd identified were, but he didn't bother adding them to the description. I paid around $300 if I remember correctly, though that was quite a while ago.
I've also been on the receiving end of information about things I've had up for sale on eBay. Whether it's good news or bad, I always add it to my auction descriptions. If it's bad news or just uncertain, I'll email the existing bidders and let them know the new information, and say they're free to withdraw bids if they want. (One time, a seller asked a question about a lens that led me to discover minor, but previously unnoticed, damage, and there were only a few hours until the end of the auction, so I cancelled all bids and then ended the acution. One bidder actually *complained* that I'd kept him from winning the auction.)
eBay can also be a good way of reaching people with similar interests. For the past several months I've been selling off a box of old fork crowns one at a time, putting a plug for the framebuilders mailing list in each ad. I could probably make more on the crowns selling a dozen of them, but what I'm really trying to do is draw in more builders to the list. Can't be sure it works, but the auctions get hundreds of hits, and so far they've always at least covered postage, and I've made a little more shelf space getting rid of crowns I'll never use ;-)
"My other bike is a car."