Colin, I bought a ton of computer stuff there. All Xbox games I own come from eBay, as well as my retro Game Gear
Never had a real problem.
How do they prevent fraud? Just look in the sellers profile for good ratings from buyers. I know some people cheat with that (since the thread here last week), but in general I think it is a good measure.
A good friend of mine collects old computers, he has every gaming computer from the 70s-80s up to now. Most bought on eBay, 0 problems.