Wash. Rev. Code § 19.86.050
It shall be unlawful for any person to lease or make a sale or contract for sale of goods, wares, merchandise, machinery, supplies, or other commodities, or services, whether patented or unpatented, for use, consumption, enjoyment, or resale, 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 commodity or services of a competitor or competitors of the lessor or seller, where the effect of such lease, sale, or contract for such sale or such condition, agreement, or understanding may be to substantially lessen competition or tend to create a monopoly in any line of commerce.
RCW 19.86.050