Any provision in any listing agreement or listing contract for the sale of real property in this state purporting to grant to the real estate broker or agent the power to execute contracts for the sale of the property shall be null and void and of no legal effect. Any contract for the sale of real property located in this state shall be executed by the owner of such property, or on behalf of an owner by an attorney-in-fact for the owner granted such authority in a written power of attorney duly executed by the owner of the property in accordance with the laws of this state governing powers of attorney.
RSA 331-A:25-g