Hi Bernd,
I don't think you'd eaven need a Schittstelle (gateway), cause there is no relevant data to share between the CMS and the shop.
Like I sayed above, client data can not be seen as something belonging in the usermanagment of a CMS, cause actually the usermanagment grantes access of different type to the pages of the CMS.
Look at the SMF forum plugin. It is installable via the addons but actually is a complete different system.
Beeing registered as user in SMF gives you no acces to websitebaker. Cause they use different user-tables.
What one could do as a shop is, f.e. take the freewebshop of
www.freewebshop.org or any other free shopping system and implement it in the same way as the SMF forum is, so installable via addons, but including the mainmenustructure of websitebaker.
I think this really is what most of us are looking for.
oh, yea and the costs of corse are 0.00 $ or €
Greetings from belgium, Sascha