I remember reading on the forums a few weeks ago that someone was looking for a used book store in Busan. A couple of friends just opened a bookstore/cafe/bar called Fully Booked in Kyungsundae and wanted to give them a shout out.
I have been and can say that they have a ton of books including the best non-fiction section I have seen in korea, all used and discounted. They have OB gold on draft, coffee, a lot of different board games (to play), and it's non-smoking inside.
Go check it out and show them that you like it when people do cool things like open used book stores where you can drink beer. Its right across the street from the chinese skewer restaurant, on the gwangali side of KSU (kitty corner from Top Mart). Look for the bright green door.