Never bought a used gun. Seriously doubt I ever will.
For me, it's like with cars: I have to know everything that was ever done with it. I can trust my firearms because I know exactly how they were fired and maintained.
I was reading a post by a guy who claimed to have had a revolver failure. Turns out on inquiry that it wasn't his gun. That it had been issued to him, and that he was totally unable to answer any question at all regarding how many rounds the gun had fired, when it was last serviced, etc. He didn't know.
To me, his report is meaningless. If you don't know everything about your weapon, then you're putting a lot of trust out there on your life-saving equipment. It'd be different if I were a collector or something, but my weapons are solely for defense and I need them to be as reliable as possible.