Unified Log-ins with external sites

Hi all,

I’m wondering what the best way would be to integrate users active sessions and info with external sites.

For example, I’m working on a bands website where users can log-on using their Twitter ID (amongst others). When they want to buy products (eg. merchandising) they are redirected to an external shop.

The user clicks on a purchase link and is send to the merch’ site basket, where the product is automatically added to the list. The user will automatically have an account created based on the same details as on the WP site. Once s/he checks-out the merch’ site will ask for payment and well as what ever else they need to know. This info would remain on their site and is not sent back to WP.

The guys (and girls) from the merch’ site are happy to create an API for me but I am curious as to how to approach this in the right way?

Thanks,
Axel.