Such bonds of a park district shall be issued when authorized under §§ 760 and 761 of this title in the name of the district, signed by the president and secretary, and countersigned by the treasurer, with the seal of said district affixed; they shall bear interest at not exceeding 5 percent per annum payable semiannually, and the principal shall be payable at such time and place as may be determined by the board, not exceeding 20 years from their date. The board of such district may sell the bonds in any manner it deems for the best interests of the district, at not less than par, and the proceeds thereof shall be used exclusively for the purpose authorized in this chapter.