The idea is good with the exception of the auto shop doing the once-over. I think they would be leery of providing a statement regarding the condition unless they were sufficiently compensated. Or unless the idea was large enough to involve a network that provided the guarantee.

What you are suggesting is in a sense what used car dealers do. Even if there is no long term guarantee on the condition of the car (many times there are though) there is an implicit guarantee that the car isn't going to die once you drive it off the lot. The point is, they charge for that because they have to cover that as a contingency if something hidden turns up.