(b) Transition rule. -- This section shall not apply to purchases of tangible personal property or taxable services in fulfillment of a purchasing agent or procurement agent contract executed and legally binding on the parties thereto prior to September 15, 1999: Provided, That this transition rule shall not apply to any purchases of tangible personal property or taxable services made under such a contract after August 31, 1991; and this transition rule shall not apply if the primary purpose of the purchasing agent or procurement agent contract was to avoid payment of consumers sales and use taxes. However, effective July 1, 2007, this section shall not apply to purchases of services, machinery, supplies or materials, except gasoline and special fuel, to be directly used or consumed in the construction, alteration, repair or improvement of a new or existing building or structure by a person performing "contracting", as defined in section two of this article, if the purchaser of the "contracting" services would be entitled to claim the refundable exemption under subdivision (2), subsection (b), section nine of this article had it purchased the services, machinery, supplies or materials. Effective July 1, 2009, this section shall not apply to purchases of services, computers, servers, building materials and tangible personal property, except purchases of gasoline and special fuel, to be installed into a building or facility or directly used or consumed in the construction, alteration, repair or improvement of a new or existing building or structure by a person performing "contracting", as defined in section two of this article, if the purchaser of the "contracting" services would be entitled to claim the exemption under subdivision (7), subsection (a), section nine-h of this article.