The commerce clause. Look at it.
The commerce clause grants Congress power to regulate commerce, not engage in it. Or are these just rather steep licenses to engage in using the spectrum. Once bought, are they transferable, without government permission and other equally steep fee, or do they "belong" to the purchaser. If the latter, Congress is engaging in commerce, otherwise they are regulating it. Regulation should be on a first come first served basis. If the licensee wishes to give up the license for whatever reason, it should go back into the pool, not be an "asset" for sale.