Which platform are you currently using? Best bet would be to use proper on-page search engine optimization for all dynamically create ads, plus injecting posting information into the title and meta description might prove useful.
We use one popular software in asp. Bad thing is it doesn't seem to be SEO at all. Will backlinks help ?
I see the following functions important for SEO friendly classifieds web site: 1. All ads should be browsed by categories, cities or some other parameters, so that search engine crawler could index all your ads. 2. Ads keywords should be included to the title and description meta tags on the the ads details pages. 3. Ads keywords should also be included to the urls of the ads details pages. e.g. your_site.com/ad_id/Home_for_sale_in_Alabama.html I'd recommend you to take a look at this script http://www.ilistersoft.com. It has all these seo features.