I want you to really think about this for a moment.

If I hypothetically have a medium house, and I want 50m gil for it, I would already arrange that sale externally.

If SE adds a system like the one you describe, and I still want 50m gil for my house but SE puts a cap of 20m on the transfer, all I'm going to do is STILL arrange a buyer externally and have him give me 30m on the side and 20m through the in-game system. If the in-game system is heavily taxed, I may transfer the house for 1m through the system and 49m around it.

Respectfully, this solution solves nothing for this who know what supply and demand really makes their plots worth.

You could follow this up with a suggestion that the GMs take a personal look at trades that happen between a buyer and seller, sure, but that's not going to happen. This is suggested like a magic bandaid for all things housing related, but it's never something that's on the table.