Yeah, but there are two sides to that (voluntary) transaction. The person bidding for labor and the person offering their labor. If the buyer and seller can't agree then the transaction doesn't take place. The value of the labor is subjective.
At the same time, we can say that there is an objective value for labour of a certain type (I'm thinking of commodified labour), which is the going market rate. Just as there is, at any given point in time, an objective value for a share of Apple Computer.
That's the whole point: "the market is willing to accept" only deals that are deemed acceptable to the two parties involves in the negotiation. It takes two to tango. Anyone is free to make outlandish offers but in order to make a deal happen it's absolutely necessary that someone willingly accepts them.