This paper describes a market-based planning mechanism used for task and resource allocation within a larger distributed, multi-robot control and coordination architecture. We are developing an extension to the traditional three-layered robot architecture that enables robots to interact directly at each layer: at the behavioral level, the robots create distributed control loops; at the executive level, they synchronize task execution; at the planning level, they use market-based techniques to allocate tasks and resources. This paper focusses on the market-based planning layer, which is comprised of two main components: a trader that participates in the market, auctioning and bidding on tasks; and a scheduler that determines task feasibility and cost for the trader, and interacts with the executive layer for task execution.