Some shops are high end, such as the many Leica specialists, and some cater more to the collector of odd old film cameras. There's even a large format specialist with a dazzling range of gear from 4x5 to 8x10 and beyond.
One of my favourite shops is Jang-shi Camera in the Howhyun underground shopping centre between Myeongdong and the Shinsegae Department Store. They sell mostly high end gear: Leicas, Contax, and also have Mamiya 7 lenses. Their website is at www.j-camera.co.kr