Come out of exit 3 of Chilseong Market subway station and hang a left at the crosswalk. (To see it on a map, check out the Green Guide.) You'll know once you've arrived:
If you don't see what you want, ask. The owners will (sometimes gladly, sometimes skeptically, sometimes resentfully) take you to their warehouses, which are often up or down sketchy staircases like this one:
In the warehouse, feel free to poke, prod, sit down on, lie down on, stretch out on, or otherwise try out whatever tickles your fancy.
Not that it's always physically possible.
Keep an eye out for the keyword: 중고 (Jung-go, "used"), since lots of these places sell normal stuff as well.
Usually, delivery fees are included in the price, provided you don't live too far away.
Remember to say 깎아 주세요! (Gga-gga chuseyo, please give me a discount!).
Best yet, you can often sell the furniture back after a year or two! Some stores will pay you almost half-price.
Don't forget to venture into the market for a snack. This bibmbap came from a self-serve buffet, included barley and beans, and cost only W2,000. What a deal!