Hello Paul,
I checked out your car listing directory and here is what I think:

Your urls[listings, categories, etc.] are not search engine friendly you might want to have them rewrite in your .htaccess file.

You are lacking a sitemap of your site so that more of yor site is indexed by search engines.

Your robots.txt file will need to point to the sitemap so search engines will know where it is.

Your link exchange networking is a link farm and google might have that network on a blacklist, maybe that is why you have no
PR.

Lose the www. from your site [can be configured in your .htaccess file] and have it drop the www if someone was to access your site with www in the url. The www. doesn't do anything for you and your website url will be shorter, cleaner, and easier to read.

Your Alexa score is low, probably because most of your traffic comes from "no follow links"

You need a better if not a custom template for your site. Something more web 2.0ish.

You might want to setup Google Analyics, to track your in/out traffic. It's one of the best stats systems for today.

You have no "favicon.ico" that appears near address bar and on tabs on browsers.

You're getting a lot of traffic, your site been up for only a few months. You must be working hard!

You seem to have found a good niche for your business.
Overall, good job but there is a lot of room for improvement that will help you bring more traffic [organic] and make your site more appealing to users as in to search engines.
Let me know if you need any help,