If it IS a true life estate, the minerals will revert to the “remaindermen” upon the death of the life estate holder. He or she CAN sell their life estate interest but when the original life estate holder dies, the interest terminates and goes back to the remaindermen. So all the buyer is getting is the life estate interest for the life of the original holder. I would definitely talk to an attorney to verify the interest is in fact a life estate, and advise you on your options.