Web service composition (WSC) is a pop topic in service oriented architecture (SOA). Proper selection of the candidate Web services is a principal task in dynamic WSC, for it directly determines the composite quality of service (QoS). But most existing services selection methods use "best-effort" strategies to serve the service requesters. These strategies focus so heavily on requester's interests that brings inefficiency to resources, because the requesters greedily compete to get the "best" service. Besides, the impartiability of a composite service should not be ignored. Therefore, an "on-demand" services selection strategy is put forward by treating the service requesters and providers as buyers and sellers in commodity-market environment. And considering the integration of a composite service, the services selection is modeling as a 0-1 multidimensional knapsack problem. Furthermore, referring to "price-demand rule" in commodity-market, the services selection model is improved for resources load balance. Finally, experiments have shown that commodity-market based service selection can bring load balance and be helpful for resources utility.