Connect Amazon webservices to my website.
The user, will be able to go into there settings page > market settings > amazon.
Then connect there amazon account (MWS credentials and product advertising api).
They would then have the ability to upload there products from my website, to there amazon account.
We would be downloading reporting every hour. (merchant/FBA listing reports, and FBA/merchant order reports).
We would use these reports to either reprice products, or show total number of orders on amazon.