The emerging traffic demand has triggered an impressive deployment of network infrastructure, including macro Base Stations (BSs) and Small Cells (SCs), leading to increased energy consumption and expenditures. However, the network underutilization during low traffic periods (e.g., night zone) enables the Mobile Network Operators (MNOs) to save energy by having their traffic served by third-party SCs, thus being able to switch off their BSs. In this paper, we propose a novel market approach 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 that is not adequate to carry the whole traffic in multioperator scenarios, we introduce a combinatorial auction framework, which includes: i) a bidding strategy, ii) a resource allocation scheme, and iii) a pricing rule. We propose a multiobjective framework to provide an energy efficient solution for the resource allocation problem and we provide extensive analytical and experimental results to estimate the potential energy savings.