For $500, even if that M12 was "put together", if the gun was solid it would still be worth it. Granted, it is a Westchester and not a superior Ithaca M37 featherlite, but it's still a good gun.
A few of you mentioned what your first shotgun was. Mine was a 37 that was a Christmas present in 1974. I still have that gun. It accounted for a lot of roosters and rabbits back in the day. Every once in a while I will hunt with it and shoot the occasional pump gun event in clays. It was a far cry better than the long barreled Mossberg 12g bolt gun with the Poly-Choke my dad let me use!!