The county board shall have power to make all orders respecting the property of the county; to keep the county buildings insured; to sell the public grounds or buildings of the county, and purchase other properties in lieu thereof; Provided, that the county board may, if it deems it for the best interests of the county, sell county property upon such terms of credit as shall be determined upon by resolution of the board; but any deferred payment shall be for not more than two-thirds of the purchase price, which shall be secured by note or notes, and a first mortgage upon the property so sold, and shall draw not less than six percent interest per annum from date until paid, the interest to be paid annually. The county board shall also have the power to sell or negotiate, without recourse upon the county, the notes and mortgages so taken; but they shall not be sold for less than par value including accrued interest. If, for any reason, such sale of the public grounds by a county board was irregular, illegal, or void, and the purchaser of such public grounds or his grantees have been in open, notorious, undisputed, continuous and adverse possession thereof for more than ten years, and during which ten years the county board has not refunded or offered to refund the purchase price, then in all such cases the county board is authorized and empowered and, when requested by the proper person, is required to convey to the purchaser of such grounds or his grantees, by good and sufficient deed without cost, the fee simple title to the public grounds so irregularly or illegally sold.