just email the highest bidder and tell him you have sold the bike privately today and there is nothing you can do.
he may be understanding and say ok-at worst he will give you bad feedback-but you can always give him bad feedback too

it happens all the time on ebay-ive had;

oh the car has been stolen
the car has been written off by a wagon
my wifes car has blown up so i cant sell this one until i get hers fixed
youre not having the car for that much!

dont worry about it-it hasnt cost the highest bidder anything,but you will have to pay final sale fee.