Hi Andy, in answer to your questions:
1. Yes, our licences allow for the themes to be used on multiple websites/domains
2. There is nothing specifically built into the theme which will share the database between different installations. You could look at a
WordPress multisite network for a shared user database or find some references via a
Google search for sharing the database itself.
3. They are using a modified version of the ClassiPress theme, but you can check out the default theme on the
AppThemes demo site to see how it looks out of the box. There are also other modified customer sites which appear in the
AppThemes showcase if you're looking to see what kind of customizations people have made for their own use.
Thanks :)