At the time and place set for the sale the board shall open any bids received in response to the notice, announce such bids, and call for oral bids. When all persons present shall have ceased bidding, the board shall declare the bidding closed, subject to the preference right of owners of contiguous land as hereinafter provided, and shall announce the amount of the highest bidder. In the event no oral bids are made and there are duplicate written bids, the highest bidder shall be the one whose bid was received first in time by the board. In no event may land be sold or a purchaser declared before the end of the 30-day period for assertion by contiguous landowners of their preference right of purchase.