Offering third party paid themes making sure theme developer gets paid

I've creating a multi-site managed Wordpress hosting site. I'm trying to figure out how to offer third party themes, like Genesis and Elegant Themes, in a way that the theme developers get paid and I can still maintain them.

Has anyone worked out a way to do that, or a deal with any theme developers?

What I would ultimately love to see some day is an in-admin system where the end user can purchase themes to use, and the theme developer gets paid and I get an affiliate commission - something like that.

Thanks.

Chuck