A sale or purchase of goods shall be deemed to take place in the course of inter-State trade or commerce if the sale or purchase--
         (a) occasions the movement of goods from one State to another; or
         (b) is effected by a transfer of documents of title to the goods during their movement from one State to another.
         Explanation 1.--Where goods are delivered to a carrier or other bailee for transmission, the movement of the goods shall, for the purposes of clause (b), be deemed to commence at the time of such delivery and terminate at the time when delivery is taken from such carrier or bailee.
         Explanation 2.--Where the movement of goods commences and terminates in the same State it shall not be deemed to be a movement of goods from one State to another by reason merely of the fact that in the course of such movement the goods pass through the territory of any other State.
         ¹[Explanation 3.--Where the gas sold or purchased and transported through a common carrier pipeline or any other common transport or distribution system becomes co-mingled and fungible with other gas in the pipeline or system and such gas is introduced into the pipeline or system in one State and is taken out from the pipeline in another State, such sale or purchase of gas shall be deemed to be a movement of goods from one State to another.]