Take line number 2 (green) to Gangnam station, start walking towards whatever exit sign you see first and you will end up in the middle of a shopping paradise. The shopping complex extends towards both ends from the central gates, so don't forget to check both halves.
Whatever is popular at the moment, age-wise mainly teens - early 30s. There are many small shops and each has slightly different style, so it is hard to generalize. At the moment, there is one very cheap, not so trendy but kinda Japanese cute style shop (if you search among the ahjumma clothing), many shops that sell "girl x sport" style, two-piece dresses, tons of very feminine and chic pieces too. There is mostly clothes and shoes, several cosmetic brands and some accessories. Everything is mostly made in Korea.
Depends on the price. The cheaper the price, the cheaper the fabric and the clumsier the sewing. The dresses I boave uneven and wrinkled hems, improperly sewn seams and threads hanging everywhere. I could probably sew them in half a day, so an experienced seamstress must be able to make them even faster. That's how you can sell them for so cheap even though they're from Korea. Also, you will rarely get the piece displayed - they will give you an item sealed in a bag and put the one you brought to the counter back. So it might be sewn either better or worse than what you saw on the hanger. Some of the shoe stores seem very decent though and accessories too. I don't think you can exchange clothes that is not relaly good quality as you won't get a receipt and most of the staff doesn't speak a lot of English. If I were you I wouldn't even bother to try, it's much easier to just mend or adjust the clothing than argue in Korean.
What to Buy
Clothing and shoes. Accessories are not super cheap, but there are some unique stores if you're willing to pay more money.
Clothing stores have almost always racks with 5000 Won items (T-shirts, shirts, skirts, shorts), 10 000 Won (dresses, cradigans, blouses) and 20 000 - 30 000 won (nicer dresses, cardigans and jackets). There are only rarely things over 50000 Won. Shoes range from 10 000 to 60 000 Won, don't get confused by similar looking styles - shoes that look almost the same can have very different quality. Accessories are generally 4000 Won for hair ties and up to 20 000 Won for headbands, 15000 - 40000 for bags. Cosmetics costs the same amount of money everywhere. Most shops will have the same price for the same items.
How does it compare to other popular shopping areas? The 5000 - 10000 Won items won't be cheaper anywhere else - this is pretty much the lowest anything can get. If the items, especially blouses go over 20000 Won, you might get similar styles cheaper in Edae or maybe Dongdaemun, since sometimes the shops there will have sales and events. So while in Gangnam blouses were moslty around 20 - 3000, in Edae some stores were having a promo and with a bit of giggling and winking I got a similar style for 15000. Yay me. I'd say shoes are a good deal too - I heard Edae was the cheapest, but I think that the shoes I got in Gangnam have better quality and even in Edae, the shop keepers were surprised when hearing how much of a discount I managed to get. So buy shoes in Gangnam after getting the price down. :)
Do not try to get the price down in shops which are busy and where shop assistants barely have enough time to unpack new clothing as the previous pices have been sold out. However, quieter shops and shops without displayed prices are very much open to haggling. I actually had no idea you can change the price, so when I was buying the first pair of shoes, I just said that 46000 is a lot since other stores sell shoes for about 15000 to 20000. The guy said that his shoes are handmade in Korea and better quality. I said "uhm buuuut...." and he said he'll sell them for 23000. Of course I got them for 50% of the original price (BTW, later on a shop assistant in Edae asked me how much my shoes were since he had similar style. When I told him the price, he went like "OMG no way they were that cheap!" So apparently I got a really good price!). In the next shoe store I was first asked 50 000 Won, I said it was way too much for me, the ahjussi asked me to write down the price I would pay, and since he was chatty and bored, I decided to play along and wrote a super low price. He said he can do 33000 minimum and I said OK. The shoes look really nice and I needed them, so I didn't care if I could get them 5000 Won cheaper elsewhere. And then in a small clothing store a lady was willing to discount 30000 Won dress to 20000 when I was hesitating, but I didn't get them in the end (I wish I did though, I might come back to get them).
BTW, I get the feeling discounts are very much related to how you look. So dress up, do your make-up nicely and style your hair.
There are no fitting rooms either, so I recommend carrying around measuring tape. Some sellers will let you try things on over your clothes, but not the majority. In Europe I am size 32/34/36 for tops and dresses and 36 for pants and shorts and 166 - 168 cm tall - and Korean free size runs a bit short and wide on me. I think you should be fine until EU size 38 (for shorts/skirts maybe 36 unless they're flowy and for shirts maybe even up to 40/42).
Shopping is a great ego booster. In order to sell their clothes, the ahjummas and ahjussis will be super nice. I've been asked if I was a model and told I was really pretty (of course it never happens outside of the shops =D). If you seem like a shopaholic they will also bring you more clothes that they think fit your style. The weirdest thing though was the shoe seller who actually put the shoes I was trying on for me. He literally kneeled down and put the shoe on and adjusted the strap to fit. I was really happy I used foot deodorizing spray in the morning and wore sandals isntead of closed shoes #Cinderella.
The Japanese-esque shop
ATM, this is a very popular store.
Gangnam style has really good prices and cute things.
33 000 Won (down from 46 000)
23 000 Won (down from 50 000)
10 000 Won
10 000 Won
Too short... in the near future I will show you a very easy way to mend overly short dresses with just a travel sewing kit and Daiso gear.
10 000 Won
Also too short and also fixed with just 2 hours of time and 1000 Won Daiso lace.
5 000 Won
5 000 Won
Express Bus Terminal Station
Take line number 3 or 7 to this station and head to exit 8. You will find ashopping place about the same size as the Gangnam station complex.
Mixed - some ahjumma stores and some Gangnam-look-alikes. More socks. There are also many flower shops, a very cute stationery/lifestyle shop (waaaay better and cheaper then Artbox) and cheap food.
Prices are around the same elvel as Gangnam station, shoes are cheaper but also lower quality. Bags are cheaper - about Edae price.I haven't bought much so I haven't tried haggling, but I don't think it would work. WHatever is 5 - 10 000 won't go any cheaper and the more expensive stores would probably not be very happy either.
Socks store - I thought the selection was same/better than Edae, but you have to buy at least 5 pairs
Pizza Mandu 3500 Won - they really taste like pizza!
Mul naengmyon or what exactly it was called. It's the cold bushwheat noodles in beef broth - 5500 Won. Really tasty too.
At the very end of one of the halves, behind all of the bistros, there is the best stationery store ever. It's much cheaper and cuter than Artbox and it actually has Korean stuff, not Japanese imports. They have everything from travel-sized hair curling irons to post-its with Hello Kitty, make-up and hair accessories.
This is it
Hope this post will help those who plan to do some shopping while in Korea. :)