Section 6. It shall be unlawful for any person engaged in trade or commerce, in the course thereof, to lease or make a sale or contract for sale of goods, wares, merchandise, machinery, supplies or other commodities, patented or unpatented, for use, consumption or resale in the commonwealth, or fix a price charged therefor, or discount from, or rebate upon, such price on the condition, agreement or understanding that the lessee or purchaser thereof shall not use or deal in the goods, wares, merchandise, machinery, supplies or other commodities of a competitor or competitors of the lessor or seller, where the effect of such lease, sale or contract for sale or such condition, agreement or understanding may be to lessen substantially competition or tend to create a monopoly in any line of trade or commerce in the commonwealth.