Exactly, but I don't think this guy is going to get it. Most people generally do seem to frown upon account selling and it's not 'endorsed' by this site or the admins. There's a difference between endorsing something and allowing something, especially something they cannot hope to or be expected to control. Also, even if account trading was banned, that wouldn't mean it's not going to happen and suddenly make trust 'meaningful' or 100% trustable.
Liability would be based on the specific facts and what a reasonable person would do. The site has mechanisms in place to moderate and threads are deleted on a regular basis. Under those conditions it may very well that the site would be expected to control those types of posts as much as reasonably possible. By allowing the posts it could be seen as an implicit endorsement of those activities. The fact that staff comes on here to ridicule those that complain about would certainly weigh heavily on the side of liability. This is why normal businesses put in reasonable controls. It is clear that hilariousandco has no concept of these issues or how they work in the real world.