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. If you ban it, no one can make an offer publicly.
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.
If account selling was banned people would just take the process off site, and unsolicited PM spam to buy or sell accounts will lead to your account getting banned anyway if the messages
are reported, but those interested in buying accounts obviously wont report them.