Once you have third parties deciding what contracts between consenting adults are and are-not void, you -- by definition -- don't have a "free market" any more... Either they are all valid and enforceable or there is no enforcement at all.
In a free market there is either "enforcement of all contracts and agreements" or "no enforcement of such". As soon as a third party gets to pick and choose which contracts and agreements are enforced, the free aspect of the free market is removed, because *really* every agreement exists at the whim of an unrelated third party.
There is no possibility of a free market without rules governing the free market which keep it free.
The only "rules" that are even theoretically necessary are ones ensuring that all agreements and contracts are enforced. ALL, except in the presence of actual fraud (which, let's be clear, is not at issue here, but is in your shopkeeper example).
Interference and enforcement are two very different things. In every scenario in every transaction in a free market you have the potential for conflict over what constitutes an agreement, about what is fraudulent, about what is intimidation, about what is theft, and ultimately about what the regulators of a free market believe is worth using force to "enforce".
I agree with the point that regulation should be working towards the most competitive free market, but I don't think a free market means what you think it means, even in the ideal.
That fundamental disagreement about terms alone demonstrates that the idea of one rule "ensuring that all agreements and contracts are enforced" itself is poorly constructed as the sole basis and definition for a free market.
And by its nature an agreement is no longer an agreement if it needs to be enforced by a third party.