Admin can set commission rates on "fixed " or "percentage" basis. For that he need to go to csmarketplace menu > vendor configuration >vendor payment section > from there admin can set commission fo all the vendors.
If you want to set commission vendor wise, product wise or category wise then you can purchase our Vendor Commission Add on.
Here is the link for Add on which will help you to understand it in more detail:
http://cedcommerce.com/magento-extensions/marketplace/vendor-commission-addon