Designer Club 디자이너 클럽

Designer's Club is probably one of the most known wholesale fashion mall in Dongdaemun.
Basement 2: Shoes, bags, accessories
Basement 1~4th floor: Women clothes
Note that the mall doesn't have escalators, you'll need to walk up the stairs.
Avoid Monday and Thursday night when it gets crowded as a lot of Korean merchants outside of Seoul come to buy goods in bundles. The owners might be too occupied to deal with regular retail customers.
9:00~10:30pm is probably the best time to go shopping before the wholesale merchants visit.
Also on Sundays, it's usually less crowded and the owners tend to offer more discounts.
Keep in mind that the sellers are not friendly. You'll need to be very strong minded in order to deal with the somewhat impolite owners. (Even usual Korean customers have difficult time when shopping at a wholesale market).

200-1 Sindang 1(il)-dong, Jung-gu, Seoul, South Korea

Every Saturday

You'll need to bargain the price with the owner but you need to buy in bundle to get the wholesale price.

Cheaper than the normal retail malls in Dongdaemun.