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.
Regardless if account selling is endorsed or not, it does not change the fact that the forum is not responsible for moderating scams (or potential scams). For the most part people are able to speak what is on their mind and post what they want with very few exceptions (obvious spam, maleware, links to illegal marketplaces, trading in things that are illegal). The only "flaw" in allowing people to trade accounts is that it potentially allows people to receive ill gotten "trust" as it would allow someone to potentially pull a scam in the future.
If a scam is being attempted with a purchased account or not, it is not the forum's job to warn others about potential scams. The fact that a purchased account was paid for would make it less likely that someone would want to scam with it unless there is a very good chance the scam will be successful and the scammer will be able to scam for more then what the account was purchased for. If you know how much a specific user's "trust" is worth in a general sense then you can be reasonably certain they will not attempt to scam for less of amounts.
There are also several legit reasons as to why people wish to purchase accounts (primarily signature campaigns). If you were to attempt to ban or police account sales then the percentage of account sales for legit reasons would go significantly down and people will malicious intentions would purchase accounts off forum, and have the purchase include VPN accounts that were previously used to connect to the forum with the account (making it appear the account was not sold). This will result in more scams involving sold accounts and a false sense of security that the account you are dealing with has not been sold.
If you were to say that you were not aware that account sales are allowed then that is your own fault for not knowing the rules of the forum that you are using.