SOS - Tanking Traffic

first use this tool to see if you are banned by Google.
http://www.iwebtool.com/google_banned

Second valid your page code and fix the errors, Google imposes penalies for bad code.

Then look at your list of backlinks. The number of backlinks is no longer highly weighted, and if you have links from related sites, spammy sites or site with a bad reputation you need to use the Google disavow tool to get them removed.

Finally the most important part.  Since May the Google weighting is now heavily favoring relevant,unique content, and all you have on the page is promotional blurbs. Content is king and until you build your pages around content you will continue to slide.

Cd&

We get data from dealerships as well as other third parties
NOT via an API

There are over a million vehicles that are listed there

The site ranking well earlier and getting good traffic, not sure what changed

There was a company in India that did some SEO a few months ago, but that was after the downward trend started

There were about 50k pageviews with 25k visitors in Dec 2012
17% returned visits
60% bounce rates

Again, I am trying to figure out what changed.

Please advise
Smit.

I am trying to figure out what changed.

What changed was the way Google calculates rankings. Most of the things that used to work are gone and some of them will even get you penalized.  The number one rating factor is is the quality of content that is unique and relevant.

Cd&

api/network of dealers. The point is there are many sites with the same exact content and you are going up against much larger sites.  

This is a good list of what changed
http://moz.com/google-algorithm-change
http://www.mattcutts.com/blog/penguin-2-0-rolled-out-today/
http://www.mattcutts.com/blog/what-to-expect-in-seo-in-the-coming-months/

What where the key phrases you ranked for?
What and when did your seo india make changes?
What other seo have you done in the past?

Thank you