That is, of course, only assuming that just the defense presents it's case. Allow the DA to talk to the jury and even with a BOS, it goes like this:
"Ladies and gentlemen of the jury, a "bill of sale?" Really? it's a piece of paper with two signatures on it. It's not a legal document. It's not notarized. How do I know that the defendant didn't get his neighbor to sign this after he printed it after he stole the gun and then gave it to him to hide all under the guise of a sale? The truth is, I don't...and neither do you...and that's why you must find the defendant guilty of theft by receiving stolen property."
A legal document does not have to be notarized.
"a document that states some contractual relationship or grants some right"
"the instrument by which a claim or right or interest or property is transferred from one person to another"