Classifieds. Can you limit the number of posts each user can make.

I am making a paid membership site and need to limit the number of posts each user can make to 50. I didnt see any options in the settings and was wondering if this is possible and if so can someone point me in the general direction to edit this. I seen a previous post that suggested bainternet plugin but i am concerned if this will limit other kinds of postings on the site (we will be having storefronts, and another sort of posting in addition to a forum)