Best way to legally convert mesos to NX

You should wait a while after the black friday sale to do this. I stockpiled nx before the sale specifically to buy the extremely good deals in the MTS due to people desperate for nx for the sale, and you can find stuff that you can get like 20k meso per nx for that haven't been sold for days. Your best bet for converting 4bil into nx through MTS is to use hard-to-find, expensive items such as attack capes, gloves, 60% shield att, etc. Anything with a large supply, such as chaos scrolls, usually gives you a bad ratio and makes you run into a lot of competition.