Korea isn't a particularly cheap country in which to pick up electronics (even for homegrown brands like Samsung), but this humongous collection of tech sellers might still appeal if you're on the hunt for a high-end gaming laptop or second-hand camera. For second-hand phones, the 8th floor of I’Park Mall is better.
Leave the train station plaza via Exit 3, turn right, then left and walk through the pedestrian overpass until you reach the first set of low-rise market buildings. Note that the area is sprawling and wearisome to navigate. It's partly closed the first and third Sundays of the month.