Sentence:  On Big Stone Lake near the headwaters of the Red River , Robert Dickson , Superintendent of the Western Indian Department of Canada , had a <WSD>trading</WSD> post and planned in 1818 to build a fort to be defended by twenty men and two small artillery pieces . 
Meaning of the word marked with <WSD> tags:  a retail store serving a sparsely populated region; usually stocked with a wide variety of merchandise