I was at a gun show and a guy handed me a 1911 and said "whaddya gimme fer this?" I looked for the serial number, it had been ground off.....
That happened to me in Dallas once, A S&W 5909 with a small piece of black tape over where the Ser. # was!! I handed it back and said "NO Thanks" should have kept it and called security? But I just didn't want to have it in my possesion!!