The increasing popularity of peer-to-peer systems has promoted the development of new techniques to support various kinds of business transactions. However, users are reluctant to conduct high value transactions over P2P systems due to the inherent untrustworthiness of peers. In this paper, we investigate building trust through automated trust negotiation, which is orthogonal to existing reputation-based approaches. Trust negotiation makes it possible to prove a peer satisfies certain trust requirements imposed by the ongoing business. We also introduce locally trusted third parties, which we demonstrate can effectively promote successful trust negotiations in peer-to-peer systems.