We require people to stick around a while before they are allowed the rights to post in BST. This way we can see they are at least somewhat of a dedicated member by sticking around at least that long.
They can build their reputation by posting in the forums and helping out that way.
Members aren't allowed to PM up until around 40 posts, I believe, so that combats some of that. But no, we can't really stop people from PM'ing offers and such before that 100 post mark.