FileMaker's API for PHP is primarily intended to display FileMaker data on the web, but it can be used to integrate with other services on the internet. Basically you would design a php page that gets the information using your e-marketplace company's PHP API, then write it to FileMaker using the FileMaker API. Note that you'd need to access this web page that you created in order to initiate the data transfer (although it could be done in a web viewer). Also, since your PHP application must be hosted on a web server, you want to make sure it's private to your internal network and/or secure against abuse from internet users who may stumble upon the page.
You must have FileMaker Server to do this.