Sentence:  Activity by British <WSD>traders</WSD> and the presence of a colony on the Red prompted the United State War Department in 1819 to send Lieutenant-Colonel Henry Leavenworth from Detroit to put a post 300 miles northwest of Prairie du Chien , until then the most advanced United States post . 
Meaning of the word marked with <WSD> tags:  someone who purchases and maintains an inventory of goods to be sold