You having value doesn't mean he or his friends want to trade with you for your value.
The problem with this argument is that your assertion doesn't happen in practice.
Besides, one scenario GP mentioned is the orphaned infant. How much labor can they offer?
Quite a bit over their lifetime.
And as mentioned above, infants can't consent to your aid. Even if somebody does decide to care for the infant, they have to infringe on the infant's freedom to do it. They had to make decisions for the infant. Even if it's in the infant's interest to have somebody make decisions for her, her freedom was still infringed.
If they can't consent or act on their own interests, then they don't have present freedom to infringe upon. Since they can be expected in the future to become human adults, able to act on their interests, then our present actions can infringe on their future freedom.
I think that's the point. The GGP's claim was that capitalism in a free market system is the most moral way to run an economy. In the scenario GP presented, they have capitalism in a free market system, yet it is not the most moral society, as we have people like you (orphaned infants) getting screwed.
No, that poster merely showed the potential for immoral action, ie, that the free market system might not, under very contrived circumstances be perfectly morally. Instead, to prove the above assertion, one needs to come up with an approach that works better than the free market in the moral sense of the original poster.
I think we can think of better examples than that. After all, the structure of almost all markets in the first place assumes the presence of immoral behavior in the participants of that market. And externalities are by definition the ways trades in markets can impose on others without their consent. Finally, what isn't traded on a market tends to be invisible to that market (unless there is an obvious proxy). As they say, money can't buy happiness.