Ang,
Did you mean proof that you purchased it? Or proof that the seller purchased it?
The other side of the coin would be you getting assurance that you were not buying stolen property.
When I got my tractor I was given a certificate of origin, along with the normal papers you get when you buy something like a tractor from a dealer. I would pass those on if I sold the tractor.
In addition to the bill of sale, I think I would want proof that the person selling is indeed the legal owner.
Just another thought.