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

Regardless of the dictatorship Nexon has over it's accounts/players, the question here is a moral one, not a legal one: Is it right or wrong to ban players for b/s NX where everything goes right?

Yes. It is and has always been against the rules. Enforced or not, people should always be prepared to face the consequences. It might not be reasonable, but it is not wrong either.