is selling housing bannable?

[Zelas]

It's getting a payment for your time and gil spent, and being in the right place at the right time.

No, it absolutely isn't. And the fact you're entertaining the idea that it's okay to think someone's entitled to extra money for their "time and gil spent" (as you call it) is baffling to me. No matter how you put it, it's a dishonest practice. Your attempt at justifying it doesn't make it any more honest.
It's simple. If you want a house, you buy it, you use it, then you dispose of it. Housing resale isn't programmed into the game. Housing isn't an item. It's not craftable or farmable, and it's not meant to be resold after the initial purchase. Doing so is 100% exploiting.

If you want to buy things and resell them higher, go back to the Market Board.