When it comes to housing, it actually is SE that is the enemy. They are the ones that control the system design and the total supply, not RMT.
If SE used a fully instanced system instead of the ward system, RMT would have no influence in housing availability. A player would tell the game "I want a LB large, here's 50 million gil I saved up" and the game would give the player a LB large. The player might still resort to RMT to get that 50 million gil (another topic) but RMT would not be able to sell the house.