Restrictions as to imposition of tax on the sale or purchase of goods

(1) No law of a State shall impose, or authorise the imposition of, a tax on the sale or purchase of goods where such sale or purchase takes place —   (a) outside the State; or   (b) in the course of the import of the goods into, or export of the goods out of, the territory of India.   (2) Parliament may by law formulate principles for determining when a sale or purchase of goods takes place in any of the ways mentioned in clause (1).   (3) Any law of a State shall, in so far as it imposes, or authorises the imposition of, —   (a) a tax on the sale or purchase of goods declared by Parliament by law to be of special importance in inter-State trade or commerce; or   (b) a tax on the sale or purchase of goods, being a tax of the nature referred to in sub-clause (b), subclause (c) or sub-clause (d) of clause (29A) of article 366,   be subject to such restrictions and conditions in regard to the system of levy, rates and other incidents of the tax as Parliament may by law specify.