Upon payment of the full purchase price, in cases where an option to purchase is given, a conveyance shall be executed to the purchaser by the chair of the board of county commissioners. Such conveyance shall refer to the order of the board authorizing such leasing with the option to purchase, and shall be deemed to convey all the estate, right, title and interest of the county in and to the property sold; and such conveyance, when executed, shall be conclusive evidence of the regularity and validity of all proceedings hereunder.