There has been recently an increasing interest in Grid services and economic-aware Grid systems both in the industry and the academia. In this paper we specify a market for hardware providers and consumers interested in leasing Grid resources for a time period. Our approach comprises a stockmarket like mechanism that enables the trading of computational power on the basis of a spot and a futures market. The spot market comprises a pair of bid and ask queues. This grid market is more complicated than the standard spot/futures markets of storable commodities, because the computational service traded in our case comprises of resources that are perishable, and has both quantity and duration specified in terms of a time interval. This is an important feature of our market mechanism, complicating considerably the trading algorithms that we develop and assess in this paper.