Pawn shops are an excellent place to shop around for a used rifle. Stop in at as many gun shops as well and look in the used section. Most people get rid of their guns because they need the extra cash not because they have shot it out.
If you are looking for a .30 cal, you can order a muzzle wear gauge from Fulton Armory. All you have to do is to drop it into the muzzle as per instructions to determine barrel wear. I got one to check muzzle wear on prospective M1 Garands and it'll check any .308 dia. muzzle.