Libertarians should give their property to Native Americans
Suppose, for example, that Jones possesses a watch, and that we can clearly show that Jones's title is originally criminal, either because (1) his ancestor stole it, or (2) because he or his ancestor purchased it from a thief (whether wittingly or unwittingly is immaterial here). Now, if we can identify and find the victim or his heir, then it is clear that Jones's title to the watch is totally invalid, and that it must promptly revert to its true and legitimate owner. Thus, if Jones inherited or purchased the watch from a man who stole it from Smith, and if Smith or the heir to his estate can be found, then the title to the watch properly reverts immediately back to Smith or his descendants, without compensation to the existing possessor of the criminally derived "title." Thus, if a current title to property is criminal in origin, and the victim or his heir can be found, then the title should immediately revert to the latter.
I found this passage interesting because it is quoted by contemporary libertarian Bryan Caplan as part of an argument for why libertarianism does not require returning Native land. Caplan reasons:
In the extremely unlikely event that any particular Indian can show that he personally is the rightful heir of a particular Indian who was wrongfully dispossessed of a particular piece of property, the current occupants should hand him the keys to his birthright and vacate the premises. Otherwise the current occupants have the morally strongest claim to their property, and the status quo should continue.
Here, Caplan is introducing a new requirement: individual ownership. It is indeed unlikely that we'll often be able to track down the individual heir of the individual Native from whom a piece of land was stolen, because historically most tribes did not practice individual ownership of land. Any land stolen prior to the Dawes Act (and some acquired afterward, since not all tribes recognized the legitimacy of the forced division of tribal land into individually-owned parcels) was the collective property of the tribe. That means its legitimate heirs are all present-day members of the tribe who owned it at the time of its theft.
I don't know exactly where Caplan's house is, but since he works at George Mason University it's plausible he lives on land stolen from the Patawomeck tribe, which is alive and well. Or perhaps his house is on Doeg land. As far as I can tell from a little Google and Wikipedia work, there are no surviving Doegs. But that genocide doesn't necessarily mean Caplan can breathe easy -- the Doegs were allied with several neighboring related tribes, so there is a plausible argument that their allies, some of whom survive today, are their proper heirs.
Caplan could reply that collective ownership is illegitimate, and only individual ownership is recognized by libertarian philosophy. Certainly this sort of terra nullius doctrine has been popular in the past. But it has no basis in the Locke/Nozick theory of property. The root of that theory is the freedom of individuals to acquire, use, and dispose of property as they see fit, as long as it doesn't violate someone else's rights. Thus, people have the freedom to create collectivities such as tribes and grant those collectivities property-ownership rights (including the right to refuse to transfer collective property to any individual).
I am not a libertarian, and thus I am not a proponent of a strict Locke/Nozick theory of property. I think Nozickian property systems are justified only insofar as they contribute to social well-being, and that property can legitimately be taken for a variety of socially beneficial uses (including redistribution, and reparations for past injustices). But it seems clear that a consistent Locke/Nozick libertarian ought to demand the return of all land to the descendants of its pre-European owners except in the small number of cases where a) the relevant tribe, and all affiliated tribes that might make a claim to heirship, have been completely destroyed, or b) the Native owners made a legitimate and fair transfer of property to a non-Native (as many tribes did have traditions of welcoming strangers and immigrants).