The thesis of Markets without Limits is that if it’s permissible to do something for free, then it’s permissible to do it for money. We have some more precise ways of stating the thesis, but that’s the slogan form.
We make some qualifications to clarify this thesis. We acknowledge that there are what we call incidental limits. For instance, if we promise to give away a guitar for free, then we can no longer sell that guitar for money without breaking the promise. But this doesn’t show that guitars are not the kinds of things that can be bought and sold.
Our argument in Markets without Limits is largely negative. By default, we presume that mutually beneficial, voluntary trades should be permissible unless we can find a good reason to conclude otherwise. We note that there are a range of objections to markets in different goods and services.