If at said election the voters, voting thereon, shall vote in favor of the issuance of the bonds, the board of county commissioners shall at once proceed to issue the same and shall deposit the bonds in the treasury of the county, the treasurer being responsible and chargeable therefor on his official bond. The board of county commissioners shall proceed to sell said bonds and deposit the proceeds from the sale thereof in the county treasury, to be paid out by the treasurer upon orders of the board from time to time as the same shall be needed. Provided, said bonds shall not be sold for less than par and accrued interest.