I have had excellent experience buying bikes on ebay. Bought 3 new bikes from large established ebay bike sellers. One of the three I bought just for the components. (I was able to buy the entire bike cheaper than buying a component group.) All three were exactly as described and very nice bikes.

A word of caution however, I have seen scam ebay posting for bikes on more than one occasion. Symptoms of the scammers: 1) only a few feedbacks, 2) postings are for 24-48 hours, and 3) posting asks you to contact the seller outside of ebay.

As long as you stick with sellers with lots of positive feedback you should be safe.