RMT is defined as "earn ISK in-game, then get something out-of-game for that (excluding more game time)". You can buy PLEX -> sell for ISK, that's fine. But not the other way around.
I don't know if you heard about the "Unholy Rage" operation. Basically CCP decided it's time to kill RMT, jumped around and banned thousands of accounts, and set up many automated systems to detect and ban RMT. Essentially, if you trade something with someone you're not connected with (ie haven't talked to / are not in the same corp / haven't talked in local), you're going to get flagged.
This is the dev blog about it.The war against the RMT element continues. We strive towards driving their operating costs to unsustainable levels. Our objective is to get rid of them, plain and simple.
This is a deeper explanation into why RMT is a bad idea. There's also the whole legal "if you let people earn real money in-game, you're a money transfer business, enjoy your AML." Entropia Universe had to jump through massive hoops for that, and I think they eventually discontinued it.