nx selling technicalities?

As far as selling NX, it is against the rules, and could get you banned. Or scammed. The way to not get scammed is to demand the buyer pay first, but of course he's just as afraid of getting scammed, and will demand you give him the NX first.

What I see most people doing is they know trustworthy sellers who are known not to scam. The problem for you is achieving that status.