As of now i dont think that the trading system is solved yet because there is many flaw in it, but i dont have to explain all that as we all already know.
Trading is a very important aspect for this game, why ? Because some class like crafting class point of interest is trading, more over for and game content where most of player need to awaken their equipment(isnt that why you created that class) that is very important to most player. And that would be bad if there is no one playing that class, i think.
So my suggestion is why dont you remove the trading restriction for high level player (like lvl 200++) and if needed create a filter program for that.
That way it is easier to spot a gold seller and i dont think there would be many bot with that high of a level and the player would think twice before selling gold, i dont think that i want to waste hours of leveling just to get banned for selling gold. For cases like someone want to quit playing and dont care about getting banned, you can always put a rule to temporary banned the buyer like a week or a month and that would just reduce the amount of RMT even more.
Wait, it isnt enough ? Why, you dont have enough people to watch over the high level player ? Then why dont you create an ingame community that specialize in ingame security like a guild of some sort. And give them an authority to kick bot from game temporarily with enough evidence while sending the evidence to GM. What, the community member might abuse this ? Oh come on… if player report the member of abusing this you just had to ask the leader to investigate and kick them from the community team.
What still not enough ? Then why dont we let another player give and extra feedback and their opinion for this matter.(Unless you valued money over player then there is nothing to discuss here)
Let us all create a comfortable place to play, please give a feedback!!
(Please correct me if there is anything wrong, thanks!)