Although an appointment in writing is necessary to constitute one an agent for the sale of real estate, one who sells real estate as the owner's agent would not be justified in retaining the difference between' the amount that he represented to his principal he received for the property and the amount he actually received, because his appointment was not in writing. Merriman v. Thompson, 48 Wash. 500, 93 P. 1075. See also Sec. 456.