Good luck with your situation.
The only way that there would be any recourse (not restitution, that would be the wrong word, here) is if there was/were any deceit, misleading statements, misrepresentation, etc.during or within the transaction.
Let's say that the buyers, KNOWING that the land was worth a lot of money, said, at some point, "that land is garbage and only good for swamp boats" or something to that effect, in an effort to cause the seller to sell quickly, cheaply or for any reason that would benefit them at the expense of the seller, they might have a case for something like "unjust enrichment" or something.
If the value of the land comes from timber, I would say they have no claim, because you can see the value, it is in your face, but if the mineral rights have to do with water, silver, coal, oil, or other unseeable things, then you can start to do you investigating. Did the buyers have any tests done on the land, and when? How did they know the land had the value, or was it a fluke? The buyers will, no doubt, claim that they knew nothing... but if you do some investigation, you will probably find out they knew because they tested the land, maybe without permission, and DID say things to entice or otherwise cause the seller to dump the property for less than its real value.