It is now against TOS to purchase a house with the intent to resell to another player (regardless of RMT or gil). The Prohibited Activities policy was updated to clarify that when Empyreum was opened.
But players who have owned and used their houses for a considerable amount of time and decide they no longer want them can resell for gil (assuming it's a FC house, of course).
The problem for the GMs is going to be proving the house was purchased with the intent to resell.