What if a problem arises? What if the either the money or the game isn't sent? What if it's a pirated game, or a broken game? Even if there's a disclaimer, these people will take it up with the site or forum officials. Since a lot of members here are minors, are they even allowed to make transactions like that? If something went wrong with a minor's order, he's going to tell his parents about it, who may look into taking legal action. While putting up a disclaimer could potentially protect PGC from legal persecution, who wants to deal with all that crap?
With all the other options on the internet, why bother? Besides, if somebody happens to mention in some other context that they're selling something, drop them a PM and set up the transaction through email or something.