It just struck me; this vBulletin board has no immediate buttons to PM people at the end of each message. Both phpBB and Invision boards typically have, depending on the forum configuration I suppose. Also, I'm unsure if the other means of contact (if the user has instant messaging, accepts e-mail etc) displays in context of the messages, but that is of limited use.
Is the PM button something that is easy to add, and would people prefer to have it? (e.g. next to the quote and fast reply buttons) Or maybe there is a display mode of this board that is changable in the settings, and I'm currently using the spartan display mode? In particular in the Trading area, I think an immediate way to PM without going through the profile might make it more clear that you shouldn't just reply to the thread with your offer or personal questions.