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

Dictionary states otherwise but I see your point. They have more in common with each other than not though so I think using them both interchangeably in this regard does not change semantics within my argument.

Actually, it could change our answers, like Heero already pointed out.
If we were to talk just from what we think, you're right and everyone's right. Morals are almost like opinions.
To answer if it's ethically right we would need more context to decide whether it's fair or not.