The focus of this chapter is centered on the network underutilization during low traffic periods (e.g., night zone), which enables the Mobile Network Operators (MNOs) to save energy by having their traffic served by third-party Small Cells (SCs), thus being able to switch off their Base Stations(BSs). In this chapter, a novel market approach is proposed to foster the opportunistic utilization of unexploited SCs capacity, where the MNOs lease the resources of third-party SCs and deactivate their BSs. Motivated by the conflicting interests of the MNOs and the restricted capacity of the SCs, we introduce a combinatorial auction framework. A multiobjective framework is formulated and a greedy auction algorithm is given to provide an energy efficient solution for the resource allocation problem within polynomial time. In addition, an extensive mathematical analysis is given for the calculation of the SCs cost, which is useful in the market framework. Finally, extended experimental results to estimate the potential energy and cost savings are provided.