A trader was an individual that bought and sold commodities.
Marco Polo was a Venetian trader and explorer who travelled with the First Doctor and his companions in the 13th century. (TV: Marco Polo) He also attended a party held by Iris Wildthyme in Hobbe's End in 1972. (PROSE: From Wildthyme with Love)
The trader Solomon intercepted a Silurian Ark and killed the entire crew. The Eleventh Doctor redirected missiles that had been fired at the Ark so they struck Solomon's ship, killing him. (TV: Dinosaurs on a Spaceship)
Sarah Jane Smith got some futuristic controllers from a Wallarian trader. Clyde Langer and Luke Smith used them to play a video game about the Battle of Waterloo on Mr Smith's screen. Sarah Jane said that she was not sure that this was quite the use the Wallarian trader had in mind for these. (PROSE: The Last Sontaran)