Honestly why should anyone give a flying fudge about used game stores that try to sell Mario Kart for 70$?
Unless you own the IPs you shouldnt be making anything off of ROMs.
At this point you cant really eliminate ROM distribution. Maybe if you flick a switch and turn off every torrent.. no wait you would have to shut down the entire internet.
If I were to make a rarity guide for Nintendo ROMs it would go something like this; on a scale of 1-10 (10 being the rarest) every single rom = rarity of 1.