I have bought things off of ebay before, but I have some strict rules. First, the seller MUST have 99% feedback or better. If he has 10000+ feedback, I'm willing to let it slide to 98%, but it must be very high. Second, he must offer some sort of DOA warrenty. I prefer 30days, but I'm willing to accept 14days. Third, although not a requirement, I look first for the "buy it now" auctions, as this is more like a store then Joe Blow. If your dealing with a store, your odds of getting good stuff go up quite a bit.
I had bought some stuff off of ebay before I had these rules and got burned. I still have no idea if its the motherboard or both Socket A CPUs that are bad. Those were my first three ebay purchases. After my third purchase is when I developed my rules and I haven't had any problems since. I have bought a 120GB harddrive (still good) and two video cards. (one died due to my gross stupidity.) The first video card was a steal. About two years ago I bought an AIW 9600 pro off of ebay for ~$70. It came with EVERYTHING. Remote, software, card, cables/dongles, everything. I didn't even realise what it was when I bought it, I just knew it was a 9600pro for <$75. I didn't even know what I bought until I was looking at the emailed package list. BTW, although I broke the card, the remote still works and is attached to my current computer. (Its a series one remote wonder.)