The City shall have the power to purchase, appropriate and own such real estate as may be necessary for public grounds, parks, boulevards, markets, public buildings, public works and other purposes necessary or convenient for the public good and for the execution of the powers conferred in this Charter or by the statutes of this State; and such buildings and grounds or any part thereof may be sold at public sale or leased as occasion may require; provided, however, no property of the value in excess of one percent of the assessed valuation of all property in the City nor any park nor any real estate used in carrying on a public utility or any part thereof shall be sold unless such sale be first approved by a simple majority of the electors voting thereon at any general or special elections.