I've bought two bike frames and a complete bike on eBay, all with good results.

The biggest issue is knowing the proper size since you can't take it for a spin. Standover height and top-tube length are critical to being comfortable, among other things. If you're experienced enough to know what you want there may not be a better place to buy than eBay.

I bought a new Fuji Team frame off eBay this past spring. It is a quality frame and a good value. The ride is a little harsh, but is tempered by the carbon fork.