All bonds issued by an operating agency shall be sold and delivered in such manner, at such rate or rates of interest and for such price or prices and at such time or times as the board shall deem in the best interests of the operating agency, whether by negotiation or to the highest and best bidder after such advertising for bids as the board of the operating agency may deem proper: PROVIDED, That the board may reject any and all bids so submitted and thereafter sell such bonds so advertised under such terms and conditions as it may deem most advantageous to its own interests.