I agree ethically/morally but at the same time, the seller likely bought them from the proper source so that creator made their money, one never knows the reasons the buyer chooses to turn around and sell them on Ebay, even if the intention was profit all along well anyone really into obtaining the NES carts either is not informed or ignorant either way buyer beware.
I don't see much wrong with it at all. The only gripe I might have is that he didn't put the word "reproduction" in there somewhere. As far as buying it from the "proper" source... well that is opening a can of worms. It is a reproduction. Aside from the original company that designed the game there is no proper source. There is nothing wrong with selling reproductions (reproductions are great), but the fact is that unless you created (homebrew) or bought the rights to a game nobody is the "proper" source to be getting the money. It is rare that somebody has exclusive rights to make a reproduction. There are times, however, when somebody buys an unreleased proto and makes a run of carts. They are going to be the only one selling the game because they are the only one with the ROM. They anted up the coin for the proto so more power to them.