Wash. Rev. Code § 36.82.100
The boards of the several counties may purchase and operate, out of the county road fund, rock crushing, gravel, or other road building material extraction equipment.
Any crushed rock, gravel, or other road building material extracted and not directly used or needed by the county in the construction, alteration, repair, improvement, or maintenance of its roads may be sold at actual cost of production by the board to the state or any other county, city, town, or other political subdivision to be used in the construction, alteration, repair, improvement, or maintenance of any state, county, city, town or other proper highway, road or street purpose: PROVIDED, That in counties of less than twelve thousand five hundred population as determined by the 1950 federal census, the boards of commissioners, during such times as the crushing, loading or mixing equipment is actually in operation, or from stockpiles, may sell at actual cost of production such surplus crushed rock, gravel, or other road building material to any other person for private use where the place of contemplated use of such crushed rock, gravel or other road building material is more than fifteen miles distant from the nearest private source of such materials within the county, distance being computed by the closest traveled route: AND PROVIDED FURTHER, That the purchaser presents, at or before the time of delivery to him or her, a treasurer's receipt for payment for such surplus crushed rock, gravel, or any other road building material.
RCW 36.82.100