No way that I know, though I'd love to experiment with Doman Access or see someone else do it with Ubercart. The idea is that you can use subdomains as a sort of content/user access control with some database table prefixing. Administration is done through the UI, though you may need to tweak your settings.php a bit.

You should then be able to setup multiple sites that share node tables but then have individual users, variables, etc. This is all in the theoretical realm for me, so be sure to post back if you give it a whirl.