You can just set the pricing to 0 to not charge: On the add/edit pages for the classified categories, make sure you set post cost to 0 (scroll down the page, when you checkmark the box to allow posts, the post cost field appears). Also on the Classifieds Config page, you can set all pricing options to 0. Later, you can change these values to charge for posting in certain categories, or for add-on options, or adding photos/videos, etc.  You may also want to start out with charging for the Featured option (Classifieds Config page).