The Open edX Ecommerce module is actually a 3rd party Django project named Oscar that the Open edX team fully integrated into the platform. Like the LMS and CMS, Oscar Ecommerce is an independent application with its own database, user list, passwords and so on. The Open edX team relies on Oauth to provide seamless authentication between the LMS, CMS and Oscar. A lot of the instructions in the official documentation are related to getting Oauth setup and working.
If you use or plan to use the e-commerce module, it's well worth your time to read it and get acquinted.