all we can do is speculate about panda. Nobody will truly understand what panda is about, its never as simple as that.
We know panda focused on low quality, shallow content websites. The only thing one can do to recover from this is to improve there site as much as they can. This means getting rid of useless no content pages, duplicate pages, low content pages and pages that offer little to no value to your visitors.
If you have an entire section or sections of this type of content, try disallowing them in robots, or moving them to a different domain as suggested by google. 300k products sounds like a lot of duplicate content issues you will need to address one way or another but duplicate content really wasnt what panda was about. You can still rank with dup content without being "pandalized"
Look at your website sections, you may have a flaw with your script that is showing a lot of useless pages thats getting picked up by google.