This is where discussions break down because there's broad disagreements on what is and is not considered "voluntary" - there are ethical/moral traditions that claim nothing is voluntary, while others that claim everything is.
We can gain clarity by defining the opposite then: Involuntary
So, what set of circumstances would be required to declare that someone is doing labor "Involuntarily?" and thus there is no "free market" for labor.
For example, would you say that the person who is manually sorting trash out of recycling for <$40,000/yr [1] is doing so voluntarily?
Maybe someone really loves that and wants to do it and would do it for free. That's possible but unlikely. Maybe someone made decisions such that this is the only job that they can get without moving or some other major life decision. Is that still voluntary simply because they are in that position? Assuming they aren't disabled, they can always put their things in a bag and walk to some place with more options. What of people that are disabled then? How much of what they do is voluntary? You get the point
The challenge then is asking, given that there is no OBJECTIVE answer to the question of what is voluntary (Extreme example: You should be constantly trying to break out of prison, otherwise you doing what the guards tell you to is "voluntary") then "voluntary" shouldn't be a discriminator on whether society has something to say about a transaction between people.
Extreme (unreasonably IMO) freemarketeers view individuals as atomic complete units and that's the end of the story. So irrespective of starting or current positions EVERYTHING is voluntary and the world is all-v-all pvp no hold barred competition. That's who is in PE and finance, so that's what you're up against.