I have a bit of paranoia about buying hardware on eBay. My personal belief is if it's new and sealed, it's fair game, but used hardware is like flipping a coin with your money. Make sure before you buy anything used or open on eBay you've examined the pros and cons throughly and are positive it's the best route to go.
The only hardware I've ever purchased on eBay was the Opteron 146 which is going in my new build later this week. It is extremely rare to find at a decent price, and there was one on eBay at $200 that was said to be new and sealed. I don't think I'd ever buy used hardware on eBay unless it was an extremely good deal, as the risk outweighs the reward.