A friend of mine bought his Yeti on EBay and is very happy with it. I bought my paintball gun on EBay and am very happy with it -- in fact, it was so clean and unmarked that I wonder if it's ever been used!

I agree that I would be worried about frame wear on any used bike, EBay or no, although I think I would be more worried about mountain bikes than road. My first mountain bike was run over, and then the vehicle backed up over the bike. The bike frame appeared to be straight (it was bent and then bent back on the way back), but I would never trust that thing on a trail. The insurance company made me give them the bike, and ever since I've worried that they might have decided to recoup their pay-out by selling the bike even though it was deemed totalled. The only sign anything had happened was very subtle paint folding / cracking near the suspension pivot; fortunately I had a neighbor at the time who was a ridiculously knowledgeable bike mechanic, and he was able to get the claims adjuster to see the problem. I think if it hadn't been for my neighbor, the adjuster would not have perceived the damage or understood the risk.

One thought, although it may not be practical -- this same neighbor, who sadly isn't my neighbor anymore, told me that you can x-ray frames for damage. Now, how one would go about getting those x-rays done, not to mention reading them accurately -- that I don't know about.