Know-How Exchange : E-Marketing

I assume you already know which 500 retailers you'd like to monitor. Otherwise, here's a few links:
https://www.chainstoreguide.com/default.aspx
https://www.internetretailer.com/top500/
https://retailindustry.about.com/

Secondly, I don't think you want this to cost much if anything. So I'd suggest to create 500 Google Alerts, one for each retailer to keep track of their activity
Parallel to this, you could sign up on their websites and receive their emails yourself

Then have an excel sheet to keep track of emails, subject line, key words.....

Mikael
B2B marketing