To be a free market, the human beings involved need to participate freely. Enslaved human beings are by definition not free, yet without them, there cannot be a market in slaves. Therefore any slave market is by definition not free.
I believe that you are making a common mistake here - equating personal 'freedom' with the economic term 'free market'.
Free market - an unregulated system of economic exchange, in which taxes, quality controls, quotas, tariffs, and other forms of centralized economic interventions by government either do not exist or are minimal
By imposing a moral layer to the definition (all human beings involved in trade must be free) you are imposing a regulation that limits the freedom of the market. But it also causes some confusing additional questions.
What exactly is freedom, and where do you draw the line? Every person has limits imposed upon their freedom. If you live in the United States you're not (usually) free to walk into the congress buildings and smear poop on the walls. You're not free to travel somewhere else (this requires government documentation, application for various permits/visas, etc.) You're not free to drive on the left side of the road, and treat red lights as green. You're not free to pay someone for sex, or to ingest certain drugs.
You aren't free to build a house on unoccupied land in the woods. You aren't free to pull your kid out of school and fail to educate them. Obviously none of these are comparable to the limits on freedom experienced by slaves . . . but my point is that there is no easy line that can be pointed to saying 'this person is free' and 'this person is not free'. There is a gradient of freedom.
So exactly how free does someone need to be in order to participate in a free market? Tricky question. It's likely part of why the economic definition of free market doesn't usually include it.
I agree that I am making a moral assertion, but disagree that it's a mistake. I do not accept the definition of free market you're espousing; I feel that definition makes a giant mistake on exactly the point I identified.
I appreciate the remaining part of the quoted post because you begin to engage in key questions about what a free market really is. I agree those are hard to draw precise lines about. Yet imprecise lines, or lines drawn with difficulty, can be better because they can express what we wisely want such a market to be. In my opinion, that's something like:
1. Participants are reasonably free to participate, and tolerably free to not participate
2. "Reasonably" here means, roughly, they have free will. Similar to how a contract is valid only if participants were not under duress.
3. Generally, participants can do with they choose, barring reasonable and hopefully minimal needed restrictions from government
4. Definite bonus points if said government is itself legitimately democratic, meaning a creature of the people rather than the other way around
5. Your US examples are very good - they are fine examples of restrictions from item 3 (I might quibble about one or two details, perhaps arguing that restricting payment for sex is unfree and not minimal or that drug laws should be looser, but clearly they're in the class that society can and does make these decisions about)
6. It's worth discussing the edge cases and whether we've achieved the right amount of "free" in our markets, but if we're in the right ballpark, there's some core of free market that we have, and it's good to have it
Balancing the burden of restrictions vs the value of needed restrictions is what democracy and, in general, good governance is about IMHO. Reasonable enforcement of restrictions against theft, extortion, and slavery are basic underpinnings of a free market IMHO.
I mean, sure, if you lived on a desert island with a small group of people, you might perhaps have cultural enforcement of norms that produced a small free market of sorts. But in large modern societies like most of us live in, there's a complexity level where in practice, we're deep into the stage of needing government and therefore trying to make it, like the of society, the best version of itself that we can.
Therefore I view free markets in a modern society not as a natural state interfered with by government, but a communal treasure created and maintained by government as a wise way to enable freedom and productivity. I do feel that economic freedom is an important subset of human freedom and therefore has a moral dimension.
I also doubt that meaningful freedom would last long without government. Bullies, guns, and wannabe slavers abound. Two of those three things are probably parts of the human condition. So we have to engage in the tough questions just to maintain a what we call a free market. Said free market will never completely fulfill a libertarian's dream, or an economist's abstraction; it will always have boundaries beyond the simplification, established through judgment and effort and maybe community struggle. I think.