For a large number of games and platforms, USED products are the entire category. Specifically with Nintendo, the vast majority of games for NES, SNES, Nintendo 64, Gamecube, Wii, Game Boy, Game Boy Color, Game Boy Advance, and Nintendo DS can only be acquired on the secondary market.
Some games have been updated for newer platforms (or even emulated on newer platforms by Nintendo to re-offer them for sale). But many, many more have not. To say that this decision on Amazon's part is not a ban against a product category because it is just banning resale of used products ... Is incredibly myopic.