I've been collecting a series of Hallmark ornaments and I spaced out and forgot to buy it for two years. So luckily I was able to find them on eBay. I have also sold a ton of books and CD's on half.com which is now part of eBay.
Oh the best score I made was a cigarette lighter charger for my cell phone. Sprint PCS charges $20 but I found it on eBay for $5. YES, it does work! And the seller even added a free reception booster thingy to my order. That, however, did NOT work.
Like someone said above, check out the seller's profile. If someone has sold 1000 items and no one has complained, I think you can trust them!
I probably wouldn't bid on something if the seller has very low feedback numbers or negative feedback that indicates a trend.