If (a big IF) the firearm was registered to you, then you should either do a FFL dealer transfer OR at a minimum get a bill of sale with noted ID numbers. The judge will allow the BOS or not, it is still evidence that you no longer owned a firearm which may be involved in a crime.
Really? And if the buyer denies that's his signature? Sorry - without a notarized signature, it's toilet paper. And if you're in Texas, NO gun is "registered to you".