Recent spectrum auction results have shown that the spectrum is usually sold at a very high unit price. Small network providers may not be able to afford it individually. Inspired by the group buying service on the Internet, group buying strategy has been introduced into the design for spectrum auctions to increase the buying power of small network providers as a whole. In this paper, we consider cognitive radio networks with multiple secondary networks, each of which consists of one secondary access point and a number of secondary users interested in accessing channels licensed to the primary user. We propose TRUBA, a truthful group buying-based auction to take advantage of the collective buying power of secondary users within each secondary network. We carefully design the budget extraction for each secondary access point within the secondary network to maximize the budget collected from the secondary users. In addition, we allow the primary user to assign its channels strategically so as to maximize its profit on each secondary network. These two features together make TRUBA significantly improve the system performance, compared to the existing group buying-based auction, in terms of the number of successful transactions (up to 105% in the evaluation results), the number of winning secondary users (up to 129%), the utility of secondary access points (up to 463%), and the utility of the primary user (up to 119%).