Why can't users add a listing?
If you are using Bondware Classifieds you will need to Create a Product in ECS.
Then you will need to Create *Categories and Sub-Categories.
Set the pricing and restrictions in the settings.
Provide a link on your site for the users to access the front end of the classifieds.
*Users can only add listings to the categories that are created by an admin. Users cannot create categories.