I believe you have to have a minimum number of posts or be a premium member in order to post in the selling/trading forums. I'm sure this is meant to dissuade people from joining this site stricktly in order to sell off items then never log in again.
Jump into some discussions in the other forums, offer some friendly advice or ask a few questions. Your post counts will go up, and then you could access any forum you wish. Or you can upgrade to a premium membership. (Posts in the Lounge don't count toward your overall post count either...took me a while to figure that one out, so I thought I'd save you the problems!!)