Think about real estate. I think it is immensely helpful for the public to have access to the sale price of houses. Imagine if the only people who had this information were a handful of real estate agencies. An individual buyer would have to go by a few scattered, probably inaccurate, whispered anecdotes. People who aren't part of an inside crowd would be at a terrible disadvantage in negotiations.
Unfortunately, that's pretty much how it goes for salary negotiations.
Also - the two consenting adults who contract privately you've mentioned, where do they turn if the deal goes south? If they feel one side has broken the contract, perhaps tortiously? Certainly, enforcement of contracts, to an extent, is within the realm of the state. I'd avoid too much tinkering, but a few ground rules to ensure an open, competitive, and, yes, transparent market, seems very reasonable to me, and very consistent with the concept of lightweight government.
In fact, I think transparency might take care of a lot of things the government tries to manage at a more granular level downstream.