Korean Job Discussion Forums

[Enigma]

My cell phone is dying (screen problems) so I want to buy a cheap used phone to replace it. I've decided against getting a smart phone, as I rarely use my phone so it's not worth the money for me.
I'll be coming into Seoul this weekend and figure Yongsan is my best bet to find a used replacement. I've received some great advice from a few posters on here, but I still have a couple questions.
1) One poster said that there's a really big electronics store next to the subway station that sells only phones on either the 9th or 10th floor which has a used cell phone section. As I've never been to Yongsan before, do any of you know the name of that store, or how to get to it from the station? Also are there any other stores in the area worth checking out?
2) I've currently got a cheap month-to-month plan with LG. I would like to stay with them and so I need to know how to identify which phones are compatible with their network. Are there any brands that are compatible/incompatible with their network?

Thanks