Part of a car enthusiast club? living on campus? belong to an international group of LARPers? want to trade with local folk musicians? ads'ere communities allow you to set up groups of people who can see and post ads targetted at them. Anyone registered user can set up and manage a group. They can be public allowing anyone to join, or private, allowing you to manage membership by confirming or denying membership requests. If you are part of a community you can post ads to that community simply when you place your wanted or sale ad.
You can use the search box on the right to look round for communities you may want to join.