Of ten ebay shipped bikes that have come to me.. I have had 3 with significant problems, two with issues, only one resolved to any degree. so, 50% batting average, and I added money to grease the wheels of commerce to get a better packing job, it is the self proclaimed professionals that did the worst.

UPS will basically want the whole lot back, if you choose that route you MIGHT get back to square zero.

If the seller is a "one time guy", does not sell often and or sells bikes even less, no hope I think. If the guy sells frequently, I have run into a situation where the killer feedback just motivates them to fold the tent, and set up a new account. ebay has been advised of this by me.. seller even had the same email address! after the change, no action.

I comes down to this, do you want the bike still? If so, its over, maybe poor feedback at the last possible momment (helps avoid rebound neg feedback). If you can't deal with it, send it back and cancel your payment.. depending on how you paid, the hoops to jump through are formidable, read the ebay dispute section, after you read it you will want to waive ebay good bye.