After the sale is closed and the treasurer has made his or her return thereof to the county clerk as provided in section 77-1813, if any real property remains unsold for want of bidders therefor, the county treasurer is authorized and required to sell the same at private sale at his or her office to any person who will pay the amount of taxes, penalty, and costs thereof and to make out duplicate certificates of sale and deliver one to the purchaser and the other to the county clerk. Such certificate shall contain the additional statement that such real property has been offered at public sale but not sold for want of bidders and shall also contain the words “sold for taxes at private sale”. The treasurer is further authorized and required to sell all real property in the county on which taxes remain unpaid and delinquent for any previous year or years.