I have a major frustration with payment providers — even contemporary players like Stripe — that can't offer a 3rd party payments system. That is, anything resembling a "marketplace" where you sell things on behalf of someone else, take a cut and pass on the rest to the content creator requires the integrator to come up with a half-assed payout pipeline. This often results in sending cheques and/or making PayPal paym…
I think both Amazon Flexible Payments and PayPal Adaptive Payments support everything you're asking for. They're separate products from Amazon Simple Payments and PayPal Website Standard / PayPal Pro which, as you know, can't be used for that kind of business.
I have a personal policy: I will never build a business that relies on a PayPal product. I've read too many horror stories about frozen accounts where people were punished for being successful to risk my client's money with them.