The director of finance shall act as purchasing agent for the county, unless the board designates another officer or employee for such purpose. The director of finance or the person designated as purchasing agent shall make all purchases, subject to such exceptions as the board allows. He may transfer supplies, materials and equipment between departments and offices; sell any surplus supplies, materials or equipment; and make such other sales as the board authorizes. He may, with the board’s approval, establish specifications or standards for all supplies, materials and equipment to be purchased for the county and to inspect all deliveries to determine their compliance with such specifications and standards. All purchases shall be made in accordance with Chapter 43 (§ 2.2-4300 et seq.) of Title 2.2 and under such rules and regulations consistent with Chapter 43 of Title 2.2 as the board may by ordinance or resolution establish. He shall not furnish any supplies, materials, equipment or contractual services to any department or office except upon receipt of a properly approved requisition and unless there is an unencumbered appropriation balance sufficient to pay for the supplies, materials, equipment or contractual services. Except as provided by the board, before making any sale he shall invite competitive bids under such rules and regulations as the board may by ordinance or resolution establish.