These are the underground shops that could be found in the "tunnel" that connects subway stations or underground pedestrians walkway. So as you are walking, you can always shop.
I really loved the colourful traditional hanbok but that would be one item I would not buy. I wanted to buy the mattress cover but I had doubt I could fit the whole thing in my luggage bag. It looked a bit bulky to be carried into the cabin.
Next time if I go to Seoul, I will make sure I will buy the mattress cover.