The Council shall establish, by ordinance, the procedure for the purchase and sale of personal property. Such ordinance shall also provide for centralized purchasing on behalf of the city. The ordinance shall also provide the dollar limit within which purchases and sales of personal property may be made without the necessity of securing competitive bids, and the dollar limit and conditions within which purchases and sales of personal property may be made without the necessity of Council approval and signature by the Mayor and the Clerk. No purchase of personal property shall be made unless a sufficient unencumbered appropriation is available therefor.