A merchant, trader, or vendor was an individual who engaged in trade. Such activity could occur in a place of business by a shopkeeper, or in any public place through direct transactions with potential customers.
Carter Winston was recognized by James T. Kirk as "a living legend, the foremost space trader of our time," further explaining that he "has acquired a dozen fortunes only to use his wealth time and again to assist Federation colonies in times of need or disaster." (TAS: "The Survivor")
Spock had the identity of a merchant imposed upon him by the Organian Ayelborne in order to protect him from invading Klingons in 2267. He noted that "Vulcan merchants are not uncommon." (TOS: "Errand of Mercy") One such merchant, Falor, was the subject of an epic work of Vulcan poetry. (VOY: "Innocence")
Cyrano Jones was a trader of such items as Spican flame gems, Antarean glow water, and tribbles. The latter caused him much trouble when he distributed them on Deep Space Station K-7 in 2268. (TOS: "The Trouble with Tribbles")
Sometimes, the profession of merchant was used as a cover by various individuals in order to conduct criminal activities. Retaya was a trader of perfumes who conducted business in places such as Deep Space 9. This activity, however, was also cover for his work as an assassin in the employ of the Tal Shiar. (DS9: "Improbable Cause")