Sections 35 to 45 of this title shall not apply to purchases of such materials, supplies, articles, or equipment as may usually be bought in the open market; nor shall they apply to perishables, including dairy, livestock and nursery products, or to agricultural or farm products processed for first sale by the original producers; nor to any contracts made by the Secretary of Agriculture for the purchase of agricultural commodities or the products thereof. Nothing in said sections shall be construed to apply to carriage of freight or personnel by vessel, airplane, bus, truck, express, or railway line where published tariff rates are in effect or to common carriers subject to the Communications Act of 1934 (47 U.S.C. 151 et seq.)