Constitution of India, 1949
286.Restrictions as to imposition of tax on the sale or purchase of goods.-
(1) No law of a State shall impose, or authorize 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 authorizes 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), sub-clause (c) or sub-clause (d) of clause 29-A 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.