Classified Ads work on volume. As a buyer, there is no point in browsing a site with no ads and as a seller there is no point listing items if there are no buyers.
This is why companies like eBay and Gumtree dominate the industry and why new entrants like Shpock are spending millions on TV ad campaigns. Then of course, you have to deal with Facebook, which has many localised free ad groups and classified ads on specialised sites, forums etc.
As Mr D says, you need a vast marketing budget to even make a dent in the above.
The other question of course is how do you intend to monetise your site?