Before we go down the road of how will this and that work absent govt, let's both conclude that we own ourselves. Can we do that? Let's assume we can...

And if we have just the market, who would exert that force regardless of its morality.
How would the market stop me from not holding up my end of the deal?

Imagine all the ways we handle these types of scenarios now, but instead of them being run by gov't funded through taxation, you get to decide what company you do business with should something like this ever happen to you.

For example, I want to sell a parcel of land, and you want to buy it. We agree on a price and agree on Tom's Mediation Services to settle any disputes should we have any. Tom's Mediation Services employs John's Security Outfit, which goes after people who reneg on contracts.
Should either of us default, these two companies would be in charge of settling the matter.

Would you really? In a 100% free society what would stop me from coming to the woods and killing you and taking everything you've built to myself? The market? How?

2 things would stop you from doing these things.
#1, similar to why (I assume) you're not one of those Steemians replying incessantly with follow4follow, or upvote me pls, or any spam comment in general, is because you have your reputation to uphold. You wouldn't be at a 56 if you did those things. Your rep here tells me that you have provided other Steemians with valuable content. Same goes in the real world, being a dick and an asshole doesn't get you very far...such as coming into my home and trying to kill me and take all my things. How many people would be willing to do business with you in the future?
#2 If I am going to give any credit to the "founding fathers", it's that they knew the people should be armed. With our current govt, or absent one, you must have some big cajones to assume that when you come onto MY property intending to kill me, that I won't pump your ass full of lead. Merry Christmas ya filthy animal.

Now, assuming you don't care about your rep and you have nothing to live for, with gov or not, that really doesn't make a difference then, does it?