Good luck getting someone at a swap meet to sign something like that. There is no reason for a seller to put himself in that situation when the next guy coming down the isle will probably buy it with very few questions.
It's pretty much buyer beware when it comes to purchasing at swap meets. You have to be knowledgeable enough to know what you're buying and willing to take a chance based on your estimate of the seller's honesty that you're really getting what you're paying for. If you want a guarantee, buy a crate engine.