In this paper we enhance the Digital Content Mediator (DCM) approach, a legitimate online service that uses financial incentives as an effective weapon to fight against online piracy. We provide needed network security support for the DCM service. The DCM mediator is a trusted notary to ensure fair and legitimate deals between digital content selling peers and buying peers. (1) In our design, the mediator sees no raw bits of digital contents. This saves storage and communication resource for the central mediator. (2) For the seller and buyer in a DCMlegitimized transaction, one wants payment and the other wants the content. The DCM protocol ensures that neither of them can stop the protocol in the middle to steal its service without serving the other party. (3) A digital content may have many legitimate copies from large amount of sellers. In a large-scale random network like the Internet, transaction fairness is defined as the condition that a buyer wants to buy the copy from the seller with shortest downloading delay (i.e., largest seller-to-buyer pairwise bandwidth) given the same amount of financial charge. DCM employs flow network security countermeasures to ensure that a seller keeps its bandwidth promises. Our experiments on the Internet confirm the effectiveness of our design.