momo, I'm not a sitebuilding guru, but I will offer my impressions, anyway.

I like that it is not as cluttered and photo-laden as many car sites one sees. Lots of whitespace, that makes it easy on the eyes.
One feature that I particularly liked was the mouse-over data on the "Latest Used Cars" photos on your home page. That certainly helps the viewer screen by model, area and price, which helps to make the search task considerably less tedious. If you intend to continue that practice on all the photos (and the "No Photo Available" listings as well, for that matter), I would predict that your site may soon be the preferred site for searching for used cars.

Perhaps it's already there, and I simply didn't encounter it, but have you considered offering the option of email notification to registered buyers, when a model they are looking for becomes listed? I suppose that could become a burdensome thing...it depends on the volume and any limitations you may have. Fully automated, though, I would think it would be a real plus for your buyers (AND sellers!).

My overall impression was very favorable, from a consumer's standpoint. It'll be interesting to watch your site, and see it develop. Don't be surprised if your competition begins to copy your methods.

...the greatest form of flattery, they say.