All lots in such city or townsite which were unoccupied at the time of the entry of said townsite in the United States land office shall be sold by the corporate authorities of such city or town, or under their direction, at public auction to the highest bidder for cash, each lot to be sold separately, and notice of such sale or sales shall be given by posting five written or printed notices in public places within said townsite, giving the time and particular place of sale, which notices must be posted for at least thirty days prior to the date of said sale, and by publishing a like notice for four consecutive weeks prior to such sale in a newspaper published in such city or town, or, if no such newspaper be published in such city or town, then in some newspaper having general circulation in such city or town, and deeds shall be given therefor to the several purchasers: PROVIDED, That no such unoccupied lot shall be sold for less than five dollars in addition to an assessment equivalent to assessment provided in RCW 58.28.090, and all moneys arising from such sale, after deducting the costs and expenses of such sale or sales, shall be placed in the treasury of such city or town.