At the time and place stated in the notice the tax collector shall proceed with the sale, commencing at the head of the list of the lots and parcels of land contained in the notice and continuing in the numerical order thereof until all the lots and parcels are sold. He may postpone or continue the sale from day to day until it is completed. He shall separately sell each lot or parcel of land, or so much thereof as may be necessary to realize the amount of the delinquent assessment which is a lien on the same, together with the penalties and costs, and fifty cents ($0.50) for a certificate of sale. If there is no other purchaser, the lot or parcel shall be struck off to the city as purchaser.