Seoul Folk Flea Market is the evolution of a former traditional market which has evolved into the one-stop Korean souvenir shop for tourists. The low building on the approach has free cultural activities for foreigners, though I haven't had a chance to participate in them yet.
The market is arranged over two floors, and generally similar kinds of stalls are near each other. This doesn't always work out, though.
The market has pretty much everything most people want. There are lots of clothing stalls including several US military gear stalls.
I don't know how to tell if they're fake, but I do know there are plenty of handbags here.
Korean vintage goods and antiques are piled on top of each other. Be aware that prices can vary from day to day and are usually negotiable.
I don't know what all the electrical cords are for. Don't expect to find standard souvenirs here - this flea market is for those looking for something different and unique. I'm sure there are some standard souvenirs here, somewhere...
Locals shop here some, too, as there are plenty of used goods and furniture.
There really is something here for everyone - even surprisingly-always-elusive baseball goods can be found if you look hard enough.
The market is open 10:30-19:00, closed the second and fourth Tuesdays each month. Some stalls open late and/or close early, so shopping during the afternoon is probably your best bet.
Subway Line 1 and the Line 2 spur go to Sinseol-dong Station. Exit 9 is the closest; backtrack once you reach the street and make the first left; it's two blocks down on the left (see my first photo). There is an amazing street flea market which, on nice weekend days, stretches all the way from Dongdaemun Station and the surrounding area to Seoul Folk Flea Market.