Traditional backup methods are error prone, cumbersome and expensive. Distributed backup applications have emerged as promising tools able to avoid these disadvantages, by exploiting unused disk space of remote computers. In this paper we propose MyriadStore, a distributed peer-to-peer backup system. MyriadStore makes use of a trading scheme that ensures that a user has as much available storage space in the system as the one he/she contributes to it. A mechanism for making challenges between the system's nodes ensures that this restriction is fulfilled. Furthermore, MyriadStore minimizes bandwidth requirements and migration costs by treating separately the storage of the system's meta-data and the storage of the backed up data. This approach also offers great flexibility on the placement of the backed up data, a property that facilitates the deployment of the trading scheme.