With the development of services computing technology, more and more voluntary services have been available on the Internet. When using voluntary services, users tend to demand higher QoS (e.g., throughput of the services) than they actually need because there is no cost. To control QoS of the voluntary services appropriately, it is necessary to design resource allocation mechanism using utilities on both service users and providers. Therefore, we have proposed market-oriented resource allocation where users and providers exchange system resources and QoS based on their utilities. In our proposed approach, service users obtain more utilities if higher QoS is allocated according to their preferences in using the services, while service providers get more utilities if their services are more effectively used by their preferred users. In order to validate the proposed method, we have compared market-based approach with demand-based approach by simulation. The simulation results show that our approach motivated users to give true demands more than demand-based approach.