is selling housing bannable?

[Niwashi]

It's closer to scalping than bribery.

And most places don't allow scalping either.

Player casinos aren't against the ToS, but people have been given warnings for spamming chat with them.

Well, gambling is specifically mentioned under Prohibited Activities:

3.6 Any Illegal Activities. You may not conduct any illegal activities whatsoever in connection with the Game. This includes every illegal activity not specifically highlighted above, including without limitation gambling, defamation, harassment, and fraud.