Backup is cumbersome. To be effective, backups have to be made at regular intervals, forcing users to organize and store a growing collection of backup media. In this paper we propose a novel Peer-to-Peer backup system, PeerStore, that allows the user to store his backups on other people?s computers instead. PeerStore is an adaptive, cost-effective system suitable for all types of networks ranging from LAN, WAN to large unstable networks like the Internet. The system consists of two layers: metadata layer and symmetric trading layer. Locating blocks and duplicate checking is accomplished by the metadata layer while the actual data distribution is done between pairs of peers after they have established a symmetric data trade. By decoupling the metadata management from data storage, the system offers a significant reduction of the maintenance cost and preserves fairness among peers. Results show that PeerStore has a reduced maintenance cost comparing to pStore. PeerStore also realizes fairness because of the symmetric nature of the trades.