I have a frame that I never ended up using for anything so I might sell it. Since it was never built into a functioning gun, I never filed an FA10 on it. Strictly legally speaking, I don't believe I'm required to do an FA10 if I sell it. However, is there any way that it can come back on me if something happens down the line? E.g. the buyer builds it and uses it in a crime, or it gets stolen, etc? Should I have a bill of sale signed by both myself and the buyer?