Section 16A. Except as provided in section sixteen H, no person shall sell or offer or expose for sale milk produced on a dairy farm, for use or disposal elsewhere than on such farm, unless as to such farm a certificate of registration has been issued by the commissioner under section sixteen C and is in full force and effect; provided, that one who purchases such milk from a dealer registered under section sixteen F and sells or offers or exposes the same for sale shall not be deemed to have violated this section unless he knows or has reasonable ground to know that the same was not produced on a farm as to which such a certificate has been issued.