Your best bet is Facebook Advertising - that is if you aren't already doing it. (pardon if you are, I quickly read through the thread)
Most of the sales coming in from automotive classifieds are from remarketing anyway. Why not leverage the same strategy, but with a wider variety of potential visitors to target?
As long as you have referral traffic coming in from search engines, classifieds, and other sources, you should be fine and dandy setting up some retargeting ads through the Facebook Ads Manager.
One thing you might want to implement is a vehicle inventory on the associated Facebook Page. So that you may dynamically serve ads to users who have viewed your vehicle inventory through your website in the past.