A better argument is that free trade based on voluntary transactions is the only just and moral system of allocating resources, in theory and in practice.
Without further qualifications, I too agree completely. However, to me it's both sad and funny how twisted the meaning of "free" and "voluntary" become when used to defend (liberal) capitalism. This is a general remark, it's not meant as an attack of how you use these words (I wouldn't really know).