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In this article, we discuss a spectrum trading mechanism implemented by the spectrum broker in TV whitespaces. TVWS are spectrum frequency bands unused by DTV, interleaved in both frequency and space. Underutilization of these bands results from the fact that the DTV transmission systems now operational in the spectrum from 470 to 790 MHz are multifrequency systems employing high tower and high power network geometries, and must be managed for interference between transmitters. We motivate the use of a spectrum broker, an entity that manages the TVWS secondary spectrum market. Such a TVWS broker's responsibilities include planning the possible broad uses of the available spectrum in the TVWS; packaging the spectrum for short-term disposal through trading mechanisms; serving the broker's customers, with spectrum-leasing contracts; and acting as the port of call to handle interference caused by its customers to the primary DTV systems or between its customers themselves. We discuss the spectrum broker's merchant and auction modes for spectrum trading. In the merchant mode, the base price is decided by the allocation procedure, which considers various factors influencing the value of TVWS in a given place. In the auction mode, the customers' demands and bids decide the final price of the spectrum. We discuss the auction design and show results of the spectrum trading mechanisms, which have been successfully applied in a real-world test scenario in the area of Munich, Germany.