I think that for almost classified websites, doing SEO is not enough to get traffic much. Besides of SEO, you need to spend some budgets on PPC campaigns such as FB ads, GG adwords,...These ways help you get much traffic quickly.
Of course, whenever you pay for traffic, the price of the ads have to go up, so the skill would be seeing how much to charge (the classifieds customer) over what you paid for the traffic.
1. create some fake id's and post fake ads. You don't want users to think the website is empty.
I don't like lying. Anyway, as an alternative, it would be good to offer free ads to fill things up and also make sure the buyers know about your traffic amounts and sources.