67.731.  Bonds, advertisement and sale, limitations on use of proceeds, retirement, interest rate — not an indebtedness of the county — bonds shall be negotiable instruments. — The governing body of the county shall provide the method for the advertisement and sale of the bonds.  The proceeds from the sale of the bonds shall be deposited in the county treasury and used only for the capital improvement project or projects for which the bonds were issued.  The bonds shall be retired serially and by installments within a period of twenty years from their date of issue and shall bear interest at a rate or rates not exceeding the rate permitted by law.  Any such revenue bonds so issued shall not be deemed to be an indebtedness of the county within the meaning of any constitutional or statutory limitation upon the incurring of an indebtedness and the principal and interest of the revenue bond shall be payable only from the proceeds of the sales tax imposed pursuant to the provisions of sections 67.730 to 67.739.  The bonds may be issued with or without the reservation of the right to call them for payment and redemption in advance of their maturity, upon the giving of such notice and with or without a covenant requiring the payment of a premium in the event of such payment and redemption prior to maturity, as the governing body of the county determines.  The bond, when issued and sold, shall be negotiable instruments within the meaning of the merchant and the negotiable instruments law.