Bonds issued by an authority may be sold at a private sale without notice or at public sale held after a notice has been published at least five days prior to the sale in a newspaper having a general circulation in the city or county, as the case may be. Bond issues sold at private sale must bear interest at a rate or rates and be sold at a price resulting in an average net interest cost not exceeding twelve percent per annum. There is no interest rate ceiling on issues sold at public sale or to the state of North Dakota or any of its agencies or instrumentalities. The bonds may not be sold for less than ninety-eight percent of par.