Classified ads sites have been, arguably, the first breed of portals with dynamic functionality. They may as well be called the first social networking sites. Some of them (looking at you, Craigslist) still thrive in nearly-original form. You can up the ante and build a classifieds site with moder design and sophisticated social networking functionality in a matter of hours - all with Dolphin and some extensions!
Dolphin includes Ads module and BoonEx Market lists different takes on Classifieds if you're looking for something special.