Property rights need to be protected or an economic system falls into anarchy. On a small scale, it is possible for the community to enforce these rights (in which case, the community has formed a local government) but on the larger, societal scale, a transcommunity government is needed (or a warlord/city boss/etc appears which is, by its nature, a form of government).

If all free trade actions could be resolved by rational, respectful, interaction of two individuals of integrity (ie they keep their word), then no government is needed. As far as I can tell, if such a world existed, we wouldn't be having this discussion.

As long as individuals need to collaborate in order to resolve disagreements, as long as certain individuals need to take responsibilities for overseeing these collaborations, government is necessary.