All sales of lands or tenements under execution shall be held at the courthouse, if there be one in the county in which such lands and tenements are situated, and if there be no courthouse, then at the door of the house in which the district court was last held. No sheriff or other officer making the sale of property, either personal or real, or any appraiser of such property, shall, either directly or indirectly, purchase the same; and every purchase so made shall be considered fraudulent and void.