I am sort of stuck on this too. From my research, you want to avoid being a 'payment aggregator' but the definition of that depends on the payment processor.

SubmitHub has a similar setup and reports using Braintree (Paypal). I wonder if he only uses that for one side of the marketplace though, because holding payments is where I'm stuck. Maybe cutting checks manually to the producer solves everything? I can't find a clear answer.