1. as far as I know, there is no restriction on selling forum accounts, it is just heavily frowned upon, you can sell forum accounts all you want, and no staff or mods will be stopping you but don't be surprised if forum members gave you a negative feedback for it.

2. if your account got banned, it is not just the account, the user itself is banned on the forum, so accounts that are connected to you personally will be banned too(at least that is how I understand it)

3. your account will stay in the forum and will not be deleted, all your posts will remain in the forum and can be seen and read by forum members.