well your answer is always a mod because your talking the forum not the classifieds product itself. What type of forum are you talking about.
However for instance if your using vbulletin 4 there are included plugins which add classifieds content to the vb
user profile as well as all the proper navigation into the vb4 navigation bar and a recent ads block on the forum home. There is also in the mod corner a cms block to display latest ads, stats etc.
Theoretically you could create a php block as well to show the latest 10 ads meaning title and description like a forum latest threads block in the sidebar its all just PHP.