BitTorrent has turned into the most popular P2P file sharing protocol and is used for various purposes such as Video on demand and Media Streaming. The fundamental problem with P2P networks in general is that quality of service highly depends on altruistic resource sharing by participating peers. Many peers freeride on the good intentions of others and BitTorrent is no exception. Current solutions like reputation systems and sharing ratio enforcement are complex, exploitable, inaccurate or unfair at times. The need to design scalable mechanisms that mitigate such problems is evident. We demonstrate through measurements that BitTorrent peers are able to barter pieces of different files (indirect interaction) which is a previously unknown property of the BitTorrent protocol. We introduce a centralized extension for the BitTorrent protocol which we refer to as the indirect interaction mechanism (IIM). IIM incentivizes seeding and mitigates problems of unfairness and exploitation while at the same time achieving linear scalability. We provide game theoretical models of the mechanism and demonstrate through analysis and simulations that IIM improves BitTorrent performance for certain cases and that it does not degrade performance for others. We conclude that IIM is a practical solution to the fundamental problem of P2P networks like BitTorrent.