Honestly, watching sunset in the city of Seoul is very hard because there are so many building everywhere and so little view. Unless you are on top of a building, mountain, or the Han River (which goes across the middle of Seoul), you are not going to see too much sunset.
I have the pleasure of walking the Hannam bridge, which goes right over the Han River, and took some photos.
What I really enjoy is the city atmosphere mixed with the river and the sunset. Korea definitely has its own kind of sunset, something I will miss greatly when I am back in the States.