One thing people buy gil for is to buy fights. Such as Titan. I've seen people with no craft leveled, they are new to the game, and yet have more gil than a veteran has with all crafts leveled and buying primal fights.

SE should make it against the rules for anyone to sells fights. All gil transaction should be made in the market provided by the game. There has to be market regulation, just like in the real world, even if people don't want it. RMT have made this game very difficult for those that want to participate in the market. The, you'll see a drop in gil buying, though it won't be enough to stop it, at least it will diminish part of it.