If you are a shopaholic like I am then you need to check out the malls around Dongdaemun Design Plaza (also known as DDP).
These malls are truly the shop until you drop places: they are open from around 9 am until 3 am (during non-COVID times, during COVID they close at the time mandated by curfew). The malls are lined with booth after booth selling wares. You look down one row and you can find twenty things you want to buy. My favorite malls are Good Morning City and Hello APM, both of which are off of Line 2.
The mall is comprised of booths, much like a flea or antique market. Each booth is run by a different owner and has different items. So make sure to shop around before buying anything, since you don't want to blow all your money at one booth.
The prices here do greatly vary. Some booths are really cheap with clothes running about 10,000-20,000 won ($10-20 USD) to more expensive booths costing 30,000-100,000 won ($30-$100 USD). For instance, I found a really cute plus-size dress and it cost 60,000 won but downstairs at another booth, I found a plus-size dress for 30,000 won ($60 and $30 USD respectively). So it really depends on the booth you are shopping at. A lot of clothes don't have tags so call out to the shop owner "얼마나요?" (olmanyo) to ask what the price is. I recommend carrying around an image of Korean numbers (such as this one here) or having your calculator app open if you are not very familiar with numbers. This is also a place you can haggle at so it is always a good idea to ask for a discount (just ask "discount?") or play hard to get to see if you can get the prices down. I am not the best at haggling but I did try on shoes and have the price go from 60,000 won to 40,000 won ($60 USD down to $40 USD). A lot of the booths have a cash discount so make sure to bring plenty of cash!
So if you are looking to revamp your wardrobe or just get a few new pieces, check out the malls around DDP for all your fashion needs!