As a platform or marketplace you will want to charge your users Fees. Fees are applied to Items and will add or subtract from the amount based on the User, payment type or Disbursement account type.
Fees are disbursed when the funds are released from Escrow, or when the Express transaction completes. Fees are refunded when doing a full refund. Proportional fees, are disbursed for partial releases and also partial refunds.