At the time and place fixed in the notice of sale the tax collector shall proceed with such sale, commencing at the head of the list of lands contained in such notice and continuing in the numerical order thereof until all the land is sold, except that he may postpone or continue the sale from day to day until the sale is completed. The tax collector shall separately sell each lot or parcel of land described in such notice, or so much thereof as is necessary to realize the amount assessed against the lot or parcel 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.