During such time as aircraft are held by a dealer for sale, exchange, delivery, test, or demonstration purposes solely as stock in trade of the dealer's business, an aircraft dealer's certificate may be used on the aircraft in lieu of a registration certificate or fee and in lieu of payment of excise tax. The secretary shall issue one aircraft dealer's certificate with each aircraft dealer's license. Additional aircraft dealer's certificates shall be issued to an aircraft dealer upon request and the payment of the fee provided in RCW 14.20.050. Nothing contained in this section, however, may be construed to prevent transferability among dealer aircraft of any aircraft dealer's certificate, and the certificate need be displayed on dealer aircraft only while in actual use or flight. Every aircraft dealer's certificate issued expires on December 31st, and may be renewed upon renewal of an aircraft dealer's license.