“ A decree of the three-judge district court set aside the Commission's order as without rational basis. D.C., 81 F.Supp. 991. The court considered as decisive that no shipments by the association were ever undertaken except at the behest and for the benefit of a member. Looking to the agency between member and association, rather than that between buyer and seller, the court saw no reasonable ground for ruling that the association was on a profit basis, or that it was holding its service out to the general public. We agree. ”