I suspect that is the situation. All new member's post must be manually approved. It is unusual for someone to pay for membership and try to post Wanted or For Sale without any other posts, so if that is a constraint of the system, we may never have seen it before.
I agree with Broch, if you make a few posts about yourself, participate in a thread or two, you should clear the manual approval stage, possibly faster than waiting for Admin to figure out if there is a problem, and coming up with a solution.
ATB
Chris