Thorgrim, you can find lots of e-commerce sites here: http://ez.no/customers/references/filter/(type)/ecommerce. Some of them have third-party systems integrated.
You can of course generate XML data with eZ Publish, by using custom templates/ extensions.
There are also some contributions and commercial add-ons around doing such integrations. For example: http://ez.no/community/contribs/applications/tinyerp_connector__1
I am as well working on an e-commerce project based on eZ. Though still struggling with some details I still believe that it is the right choice. :)
