Free-space optical (FSO) communication offers wireless connectivity with high data rates and low system complexity; however, it suffers from the infrequent adverse weather conditions. This work proposes a novel hybrid RF/FSO system based on a game theoretic spectrum trading process to enhance the reliability of FSO links. The proposed system is considered to be both spectrum-and power-efficient. It is assumed that no RF spectrum is preallocated to the FSO link and only when the link availability is severely impaired by the infrequent adverse weather conditions, i.e. fog, etc., the source can borrow a portion of licensed RF spectrum from one of the surrounding RF nodes. A market-equilibrium-based pricing process is proposed for the spectrum trading between the source and RF nodes. By using the leased spectrum, the source is able to establish a dual-hop RF/FSO hybrid link to maintain its throughout to the destination. Our extensive performance analysis illustrates the effectiveness of the proposed communication system. It is demonstrated that the proposed scheme can significantly improve the average capacity of the system, especially when the surrounding RF nodes are with low traffic loads. In addition, the system benefits from involving more RF nodes into the spectrum trading process by means of diversity. Finally, the application of the proposed system in a realistic scenario is presented based on the weather statistics in the city of Edinburgh, UK, which demonstrates that the system can substantially enhance the link availability towards the carrier-class requirement.