In this paper, we considered energy trading within a wireless power transmission system. Considering one energy hunger sensor device and battery operator, the battery operator uses electromagnetic wave to transfer power to the sensor device. In the sensor device, a rectifier is used to convert such electromagnetic wave into DC power, and this rectifier is an important part of deciding the actual power transmission efficiency. we considered the nonlinear rectifier model and discussed the problem of optimizing the power to be transmitted. In addition, considering the price per power, we considered the game between sensor device and battery operator.