Drupal Services module integration for shared session

I have a Drupal site where users will create an account. Once they log in to the Drupal site, they can visit an external ecommerce site where they can buy tickets for an event. If user logs into drupal site and then visits the ecommerce site, user needs to automatically be logged into the ecommerce site with Drupal credentials (username, pass etc).

So Drupal site needs to communicate with external ecommerce site to authenticate user.

In addition, each user account should hold custom data fields (city, state, etc). Perhaps this can be done with profile module.

Please email arthur@rootdownmedia.com if you have any insight or are interested in developing something like this.

Thanks!