Charging for labour is profiting, profiting by selling something that you don't own the rights to is bootlegging. I don't see a problem if the makers of "reproductions" only charged for parts, supplies, and shipping, charging for labour is what's shady when your just stealing someone else's game. If the games were actually released in the public domain then go nuts and charge whatever you like.
Plenty of people are willing to pay $3 for bootleg DVDs sold at flea markets, it takes time to film those in theaters, or download and then burn those so they're just as legitimate as "reproduction" games.
Still, if you want a separate cart then go and get one. Just don't pretend that it's perfectly legal when it's not, companies choosing to not enforce it isn't making it legal.
Technically you can just use a flash cart for homebrew games so they're slightly more legitimate than a straight forward "reproduction", if you choose to use one for piracy then it's your fault. Just like choosing to use a kitchen knife as a weapon would be your fault, there's nothing wrong with a kitchen knife on it's own.