If a gun is still in proof, there is no real reason to have it reproofed. Proof is proof. I suspect anybody who would do such a thing, have now had their fourth booster shot and wear a mask when driving alone in their car. A dealer or auction house has absolutely no incentive to dump out of proof guns because the cost of doing so and getting caught is far beyond what little they can gain for doing it. Now if the gun has some historic value and deemed not likely to pass reproof and will not be shot, there use to be a proof exempt certificate. Do not know if they still have them as a option.

Think of what selling an out of proof gun would mean to a dealer who sell several thousands guns a year. A few pounds gain against a real inquiry, a fine and thousand of people finding out their reputation for selling good firearms is false.