EVE Online and a number of other MMOs have "legalized" goldselling in the form of "Plexx", "Credd" or another similar item.
It works like this:
1. A player buys a month of game time in the form of an item from the FFXIV online store.
2. They can then sell this item on the Market Board in game. Other players buy it with in-game gil: NO NEW GIL IS BEING "CREATED" HERE!!!
3. Since this costs SE absolutely nothing, it allows them to "legally" sell gil, thus making RMT sites unappealing by comparison because there's no risk of being banned by buying game time from SE.
Seriously, I spend most of my time on /busy to avoid the constant RMT whispers. This would put a big nail in the RMT coffin and make SE more money to boot...