Something like the following:
I [owner's name] owner of a 19__ [brand] [model] with a VIN of ____________________ hereby agree that I have sold said vehicle to [buyer's name] for the amount of $__________________.
Signed _____________________________ on this __ day of _________________ 2002
You will want to remember to have it notarized, otherwise it becomes a real PITA. Experience talking there.
ZV