Well I have no problem buying used carbon from ebay or any other format. I've done it before and am sure I will do it again. I just want clear pictures and lots of info on condition and parts on the bike. That auction doesn't have any of that. I would ask the seller a bunch of questions and for better pics of anything i might be concerned with. He has not sold a bunch of stuff before but does have a history of bike items so I would feel ok about that. This bike was sold before but must have fallen through for some reason. I would ask about that too.
His starting price and BIN seem within reason if all questions are answered to your satisfaction. I don't think msrp was ever $3800 for that bike though, but maybe.
If you know what fits you and are informed on the product then ebay can be a great resource for used bikes. Just ask q bunch of questions.
I do have a similar bike (Trek 5900) up on Ebay right now but it's not the correct size for you if the above bike is your size. It's been a while since I have sold anything on Ebay but I tried to give a bunch of info on condition and more than enough pics to help any prospective buyer obtain enough info to make a decision.
http://cgi.ebay.com/ws/eBayISAPI.dll...ht_1713wt_1141