Mate, you can only tell the eBay guy the truth, at the end of the day if it goes past (6 hours is it) you cant stop it anyway...

Just say when you last checked it was nowhere near the price so you didn't expect it to go, you had a deffo offer which you took, and then the bids suddenly ( like they do on eBay) went mad, but you could do nowt about it.

If the bike aint there he cant collect it can he?