Got another idea. Actually, 2.
#1) Take the current inventory, grade them off. Anything with a muzzle wear of, oh, lets say 4 (doesn't have to be that, just follow me), or a TE of 6 (again, just grabbing a number out of the air). Put all those rifles in a bin. Take what is left of the non-collectors ones, and set them aside. The better shape ones go for competition shooting. Put a limit of 1 per year, 1 every other year, or something like that. Take the worse off ones, and sell like they currently are selling. Same for the collectors, and they bothe count towards your total. That would ensure that at least some of the good ones get left out for competition.
#2) A simpler idea. Keep everything the same. First rifle and second rifle, you pay the price advertised. Third, you pay 10% more. Fourth, 20% more. Fifth, 30% more, and so on. That would at least slow some people down.