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

Just going to say this right now, Blizzard suspends people who trade real world goods for in-game currency, or vice-versa. I don't see why Nexon wouldn't do the same thing if they were actually competent.

I find this comparison amusing, considering Diablo 3 was practically built around the concept of paying real world money for virtual goods from other players on a scale so large it's not even funny.

The only difference between doing it in the market system and doing it outside of the market system is that Blizzard does or doesn't get a cut depending on which it is.

As for the fairness of banning players for it, yes. I believe it is morally fair. They clearly have stated it multiple times that you are not to do it, and you will be banned for doing it. Is it morally right? Well, that's a different story, and really, up to huge debate. I believe there's the huge situation of worrying about scammers that any MMO publisher has to deal with, and to ban the practice outright is a pretty understandable thing.

The true answer is pseudo selling, where you can put NX items into the market. My understanding is that GMS doesn't allow this at all. JMS has limited allowance of this, but I know that KMS has basically free reign in selling NX items in shops. I believe that allowing that is as good as it would get, but free buying/selling is a scammer's playground, and I understand completely why it is a bannable offense.