This is a place where you can find all kinds of big fashion brands (Such as Nike, Zara, H&M Uniqlo, Mango, JoeFresh) and Korean cosmetic brands (The FaceShop, Missha, SkinFood, TonyMoly, Holika Holika, Too Cool  For School and more). There are lots of small and large Korean retail stores full of trends & lots of cute fashion accessory stores, so  you might also want to to hop into these stores to look for real Korean fashion style.
There are also Lotte Department store right in MyeongDong where you can enjoy hands free (buy and pick up at the airport) duty free shopping.

Dong Dae Mun

This place is the mecca of Korean Fashion! Most of the small fashion and accessory retail stores you found on the streets are mostly all from the Dongdeamun merchandise wholesale. Dongdaemun is cheap, but you can’t tell it is always the cheapest place because, the price of same items that you can find in DongDaeMun are different by each shops. Even Koreans do bargain if they go shopping to Dongdaemun.  Remember, Dongdaemun is a night market so go there after 9 PM to enjoy the night!

Hong Dae