If you buy land where the only access is through another person's private property, that is your own fault. The purchaser has a responsibility to do their own due diligence.

I was thinking more the opposite; You could already own the land and then someone else buys up everything surrounding it.

Obviously, we're leaving LibertarianLand for a moment, but the law generally makes it impossible to split up a parcel of land in such a way that one of the lots will be landlocked. If that lot is going to be landlocked, they must create an easement allowing travel across the property that does have access to the road.

Similarly, easements don't go away when they change buyers. For example, if you own a landlocked property, and you have an easement with your neighbor that allows you to drive across his property on his private road to reach yours, and then the neighbor sells his property, you can still use that road - the new owner cannot say "tough luck."