What transaction are you referring to not being voluntary? Transactions in the economic sense are by definition voluntary. Without a monopoly on law, people hire courts of their choosing to settle disputes. They are not compelled by a third party to settle disputes in the third party's court. Most importantly, they are not compelled by a third party to use the third party's law. They may each have their own laws even.
More about polycentric law: http://www.tomwbell.com/writings/JurisPoly.html
No, they aren't, which is why we distinguish with the term "free market" the platonic ideal of an economic situation in which all transactions are fully voluntary.
> They may each have their own laws even.
If each party may have their own laws, those aren't actually "laws" in any meaningful sense.
> More about polycentric law: http://www.tomwbell.com/writings/JurisPoly.html
I find it interesting that that article highlights exactly why the historical systems described as polycentric legal systems (which are not, in any meaningful sense, a "free market of laws") tend toward natural monopoly and solidification into state law, apparently without realizing it, stating that defectors from such a system "would then have to either accept the jurisdiction of its courts or suffer ostracism from the community of law-abiding folk."
Of course, that effect not only applies to accepting some of the available choices in a polycentric system, but also accepting both the particular "law" (a rule system that becomes binding retroactively only after a dispute occurs and a forum chosen is hardly "law" in the normal sense, but...) and particular courts preferred by the most powerful groups in the community.
Huh? Law is enforced whether polycentric or monopolistic. I don't see what that has to do with the evolution of monopolistic statutory law.
Most likely, both.