This process highlights the Refunds / RMA workflow. This is the default process where the Vendors are responsible for approving the products from the customers. The refund process involves all parties of the transaction generally, as each role has different accesses and responsibilities.
Customer: They are responsible for initiating the refund, and if required to respond to any questions about the refund/product in question.
Vendor: Generally they are responsible for product expertise, and approval of the refund to be processed.
Marketplace Owner: The MO is responsible for actually paying the customer back the funds (creating a credit memo). The reason for this is that the MO is the one responsible for the initial payment reception from the customer, therefore the customer needs to be paid back with the same payment method.