I would love to hear the results of a competitive buy-back. Maybe a gun club could make a better offer on the same day and compete with the police.
My biggest concern would be the stolen firearms. I think the first stolen firearm you purchased would open a can of worms and you would not be buying anymore. When I ran an auto repair business (Cars, not guns) I would get more offers to trade the work for this gun or that gun. I picked up quite a few nice guns doing that. I also had the police chief check the guns to see if they had been stolen. Quite a few were. The town had a policy that they would go after the guy who sold, or traded me, the gun that turned out to be stolen. The police chief told them they would have to pay me for the work I had done, or they would be charged for that also. (Theft of services or something like that.) It was a pretty nice arrangement and he, the chief, would get to purchase what I did not want or need.