This paper is about using intelligent agent technology to facilitate the C2B (Consumer-to-Business) business model on Internet. The benefits of C2B, a collective purchasing model, are obvious, but there are some unresolved problems that obstruct the growth of this model. For examples, how to gather together individual's preferences and translate them into a group's synthesis? What is the mechanism to interact with each other in the virtual group, and how does the group negotiate with the sellers? In order to resolve these difficulties, we have proposed a multi-agent framework to support the C2B model in electronic commerce, and a new algorithm - adaptive-AHP has been proposed. Finally, we have implemented a prototype system on NET for notebook computer purchasing to show how the adaptive-AHP is applied in the multi-agent systems.