I have bought (or tried to buy) several bikes on eBay with mixed experiences:

The good:
1) Older Dave Scott Centurion Ironman (first bike specifically built for triathlons) ... good price, easy transaction, bike as described. Seller was someone cleaning out the garage and did a very good job.
2) Seven ti road bike ... great price, outstanding service from communication thru shipping and after. Seller was a friend of the estate (bike's owner had died of breast cancer) and worked at a bike shop. Super guy, super bike and I always find an inner reserve of strength when I think of the bike's original owner and see that Ironman Kona sticker on my tube.
3) Co-Motion S&S coupled tandem ... great price, great service and fast shipping (even when the cost turned out to be far more than seller had anticipated).

The bad:
1) DeRosa King (for my husband) ... was high bidder but didn't meet the reserve price. Received a "Second Chance Offer" from seller and accepted / paid. As you'd know if you read eBay's help, second chance offers are at the bidder's max price. The seller didn't understand this and tried to extract hundreds of extra dollars from me. When that didn't work, he cancelled the sale. Yes, I eventually got my money refunded but I didn't have access to the funds for five days. The eBay Resolution Center can't force him to honor the sales contract, and I filed a Seller Nonperformance report against him. He thinks I was "trying to take advantage of him." I think he didn't bother to understand how eBay works, and made an offer that he refused to honor.

2) I'm not bidding right now on the replacement to the above - a Cervelo Soloist - because I fear the same inexperience level in the seller who has only one feedback rating. He sold the bike for $2,000 and wrote a positive review of the buyer for his prompt payment. But now less than a week later he has the bike relisted (and with a starting price of $2,100). I've asked him to explain why the deal fell through but haven't had a response. I'm staying a mile away from that seller.

So as a few other people said, ask questions not just to ensure it is the bike you want but also to judge the service, knowledge and attitude of the seller. You can predict a lot about the service you'll receive just by asking a simple question.