People selling accounts is impossible to prevent, which is probably why there is no rule against it.
Well, in theory it should be possible to ban it.
How? The "rule" of "selling is acceptable" is because the admins think its not possible to ban or catch sold accounts.
If you ban it, no one can make an offer publicly.
While the threads are public, the names of the accounts are usually kept hidde AFAIK.
If someone makes an offer via PM, those PMs can be reported to moderator and it can lead to a ban on who's making the offer.
Unless both parties want to trade.