Must admit I've only once bought a shirt from a Korean shop, but they were the very example of perfect service actually. They made a mistake with the postal address - I provided them with the correct address but they went with an old address in my Paypal details I had forgotten to change. Mailed them after a couple weeks that the shirt had never arrived, they send a new one free of charge. Then I picked up some mail for a friend at the old address I used to live, found the initial shipped shirt there. Got back with the shop, they told me to consider it a gift and stated the hope I'd shop at them again.
Regarding the sizes, it's a bit of a hassle, but usually they provide inches/cm for all the important things (overall length, sleeve length, shoulder, etc) so what I do is measure clothing I have and fits well and compare with their charts. Haven't had problems so far. A thing to be more cautious about is how some clothing of course is wrapped around a mannequin 'perfectly' - you'll never be able to wear it like that and walk around
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The thing I like about it is basically that there's really a small chance you'll see someone else walk around in it - opposite to Zara/H&M/We - brands with affordable stuff I see everyone here walk around in. I actually get tons of compliments for that single Korean shirt I bought simply because it's a little different from everything available in shops here.