It's obvious why most libertarians bristle at issues concerning justice in appropriation. If you didn't create something, and have never even used it, then how could you possibly have a legitimate complaint if you found it to be the property of someone else? No one is detectably made worse off when someone encloses a formerly unused piece of the commons. Perhaps private appropriation of commonly owned property, like that discussed by Holcombe and Long, would be dispossession, but how could we say the same thing about something which was never so much as touched by the allegedly "dispossessed" parties?
It does seem that Locke rested on the idea that the world was "given" to us in common when he casually tossed in his "proviso." If one takes a skeptical view of religion, as I do, then it seems fair to say that we have no decisive proof of the fact that the world was in fact "given" to us in common by anyone or anything. If the world was not given to us - if we were never collectively declared its owners - then how could we possibly argue that it belongs to all of us in common?
I'm honestly not sure how to respond. And if anyone has any ideas, I'd love to hear them. But it seems to me that the idea that the world belongs to all of us is not completely empty. I'm not sure what it means to say that the world belongs to all of us, or why we should believe it to be true. But as far as moral intuition can serve as a good starting point, it does seem to me that any theory which denies outright the claim that we have no right to a share of nature's "gifts" cannot be right.
If, in an anarcho-capitalistic society, a person were ejected from her parents' land, and had nowhere to go because all of the land was privately owned and no one wanted her, then it seems absolutely wrong to say that she should just die or somehow vanish. It doesn't avoid the question to change the story so that someone would want her. That sort of evasion can't anchor a philosophy. Perhaps everyone in the area knows and loves the girl's parents, and wants to honor their decision by refusing the girl a place on their land. Doesn't the girl have the right to demand that she be granted access to some place to live on the surface of the Earth?
I can't say how it could be determined what place she ought to be allowed to go, or whether she should be able to own such a place (that is, exclude others from using it), but it does seem like a proper theory of appropriation would include some assurance that the girl would have somewhere to go. I don't want to get ahead of myself, so I'll stop there.
But what could justify this sort of intuition? That is, we don't seem to disrespect the girl by originally appropriating the land, and we don't seem to be disrespecting the girl when we refuse her access to our property. At least, if taken individually we wouldn't think any of our actions to be disrespectful. Perhaps we might want to consider this as an emergent problem? That's a can of worms to be opened later, but I do think it's a promising possibility.