Would it be morally fair of Nexon to ban buyers/sellers of NX? I think not.

If you're trading NX for items or vice-versa without scamming/hacking/cheating, and Nexon bans you, that's completely unreasonable of them regardless of what their rules say.

Except enough people have been victims of such practices. Guess what? Most of them end up whingeing in-game or on the internet about how 'Don't buy NX from XXX he's a scammer!!11!'. The rule is there so that if anything goes wrong, it is not Nexon's responsibility.