Taking advantage of the New Joomla! features of Access Control System (allows site administrators control over who can view and manage content), we designed a component that allow you to automatically create/update user's groups and status.
Combine it with any other extension that utilize ACL, and you can achieve anything. For example we use this component to sell our extensions!
It's ability to add the selling buttons anywhere (articles, modules etc) where the Joomla! editor is used, gives the incredible flexibility to create any kind of store you wish, utilizing your site template(!!!) and nothing more.