I think you can actually sell it through maplestory.
There was a system called MTS or something which was basically an area to trade NX and ingame money freely, but they had problems and closed it down until they could fix the system, which seemed to mean indefinitely. So you can't.
You can't transfer NX, but you might be able to gift with that. 5k isn't that much relative to the good, permanent things you'd want, but it's worth something. I'd like it, but...