The Purchasing Agent shall purchase all supplies for the City, and approve all vouchers for the payment of the same. He shall also conduct all sales of personal property which the Council may authorize to be sold as having become unnecessary or unfit for the City's use.
   All purchases and sales shall conform to such regulations as the Council may from time to time prescribe; but in either case, if any amount in excess of $500 is involved, opportunity for competition shall be given. Where purchases or sales are made on joint account of separate departments, the Purchasing Agent shall apportion the charge or credit to each department. He shall see to the delivery of supplies to each department, and take and retain the receipt of each department therefor.  Until the Council shall otherwise provide, the City Manager shall act as such Purchasing Agent.