is selling housing bannable?

[Zelas]

So the same would apply to run/pony sales, off MB sales over 1M gil, player casinos, and any other player created exchange.

But in the end there's a nice catch-all in the ToS, saying it's their game, they do what they want.

Again, similar but not entirely the same.
I don't condone selling runs, because I think it's a pretty stupid practice and promotes laziness, but at least, there, they're selling something. The person actually GETS the kill/drop/whatever, and if they don't, they have sufficient evidence they got double-crossed (through screenshots, chatlogs, etc.).

For housing, you're not paying the player for anything at all other than a sort of bribe ("I'm holding this house hostage until you give me back the money I paid for it."). You're technically being asked to pay for the seller's purchase (or sometimes, several times their purchase) on top of your own. You pay for THEIR past entertainment/use of the house, and then buy the house AGAIN for yourself. Which you could otherwise get for only one of the fees if they were being honest and just let it go as they should.