Accept the bankers draft on the condition he accompanies you to your bank to deposit it into your account prior to him loading up bike or riding away on it.
If it's genuine, it will clear pretty much immediately on deposit. If it's fraudulent, he simply won't agree to going to the bank.
You got a bank local to you? I know we don't all have one handy.
x2. I am sure a bankers draft means it clears immediately, so you will know there and then if there is a problem or not.