Hi
As for now only hosted version of ShareTribe splits fee from payment made to merchants.
There is a tutorial how to activate paypal on OpenSource version for selling but under condition of not
applying such fees.
So the idea to solve this, is to automatically create PayPal invoices to merchant with cumulative fees for a period of time (month/quarter) via PayPal REST API, calculated from number of sold items and their price.
The status of invoice can be checked so one could automatically make merchant inactive in case of not paying for their activity.