The county purchasing agent shall, subject to the control and supervision of the board of commissioners, (a) purchase or contract for all supplies, materials and equipment, and contractual services required by any Office, department, institution or agency of the county government subject to the provisions, restrictions and limitations of this Division; (b) enforce standard specifications established in accordance with this Division which shall apply to all supplies, materials and equipment purchased for the use of any Office, department, institution or agency of the county government; (c) have charge of all central storerooms now operated by or hereafter established by any Office, department, institution or agency of the county government; (d) transfer to or between the various Offices, departments, institutions or agencies of the county government and trade in and sell supplies, materials and equipment which are surplus, obsolete or unusable; (e) have charge of such other purchasing activities as the Board of Commissioners may assign to him from time to time; (f) distribute or cause to be distributed to the various Offices, departments, institutions or agencies of the county government all supplies, materials and equipment purchased by him. Except as otherwise expressly provided by law, no supplies, materials or equipment or contractual services shall be purchased or contracted for by any Office, department, institution or agency of the county, or by any officer or employee thereof, but all such supplies, materials, equipment or contractual services shall be purchased or contracted for by such county purchasing agent in accordance with this Division.