Any and all bids or proposals under this act shall be accompanied by a good and sufficient bond or a certified check in such sum as the said purchasing agent may require, the same to be stated in the advertisement as aforesaid, and the said agent may, if he deems it advisable, advertise for the various articles and supplies needed separately or together, and may accept the bid or bids for the same to be furnished separately or all by 1 bidder, as may be most advantageous to the county, and when purchases are made by the county purchasing agent preference shall be given to county or home products, all things being equal.