It sounds odd to me that the buyer placed a bid on the bike and then asked you for the serial number. Personally I would have thought that if a buyer had a genuine reason for wanting to know the serial number - for example, to run a check in order to ascertain that the bike wasn't registered as having been stolen - then he/she should have asked you the question with regards to the serial number before placing the bid, not after. Having said that, however, it could be a genuine mistake on the part of a newbie who saw something that he/she had been looking for for ages, got caught up in the excitement of having finally found what he/she wanted and immediately placed a bid for the bike and subsequently thought "Oh, dammit - I should have asked about the bike's serial number to check it's legit."
If you feel uneasy about the buyer's question then you have one of two options. Either don't answer the question and hope that the bidder either retracts his/her bid or ends up losing the auction if the bid isn't retracted, or alternatively you could cancel the buyer's bid and add his/her User ID to your Blocked Bidders List. However, the latter option would be a bit unfair if the newbie isn't out to screw you over and simply just forgot to ask questions before rushing to place a bid.
At the end of the day you really need to trust your instincs. Do you feel that this is just an oversight on the part of a newbie to eBay, as given in my example with regards to the possible reason as to why you received the bid prior to the question about the serial number, or do you feel certain that your being set up for a scam? If you feel uneasy about the matter then trust your instincts. Given the amount that the bike is going for I'd hate for you to lose both the bike and the money if the buyer were a scammer and succeeded in winning the auction and screwing you over, although having said that the buyer may genuinely just be a newbie who isn't quite familiar with how the site works yet and who subsequently appears to have got you quite worried as a result. The newbie may not even win the auction, in which case his/her bids could help to push the final selling price up quite a bit, depending upon how many times he/she decides to bid for the bike before finally being outbid.
