It really does not help much with seo. It will ge a new site or page within a site indexed quickly if links from a high pr classified site are placed.
But most algos have discounted classified site links because they know that they are temp. Usualy no longer than 45 days.
There is also another factor involved, Even if you had placed 1,500 ads on a classified site, they would all be from the same IP address and that would further discount the value of the links and could also catch you in a filter for buying sitewide links.
Lookup "google bowling" to read about the effects. This is what some spammers use to eliminate top sites so that they can get top positions easily.
As long as you post within guidlines of the classified sites, you will not get into trouble, but will not help much at all.
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