I was looking into this very same scenario, and I believe I've found that it is supported via PayPal's Website Payment Pro API. It appears as though there are several available use cases, such as:
- Taking a cut, as you described
- Dividing up a single customer payment among multiple payees (so if you have a shopping car where you resell items from several different providers, you can divide the payments up based on who provided each item)
Take a look at the document here and see if it fits your needs. I'd be very curious to hear how your integration goes, since I'm looking at something very similar for my forthcoming site.
https://www.x.com/docs/DOC-1328
EDIT: You should also take a look at the Adaptive Payments API. That was the other service I found that may fit this use case...
https://www.x.com/community/ppx/adaptive_payments