Not sure about the areas you mentioned but there used to be several shops selling used phones in the Seomyeon underground shopping mall. I haven't taken a walk there for about 18months, so may not be there anymore.
Or if you check the classifieds on this site people are selling their phones all the time.
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http://www.facebook.com/pages/ISAIK-Incorporated/611653045529104?ref=hl
I hope this information , can help you ^^
Cheers
Not sure about the areas you mentioned but there used to be several shops selling used phones in the Seomyeon underground shopping mall. I haven't taken a walk there for about 18months, so may not be there anymore.
Or if you check the classifieds on this site people are selling their phones all the time.
www.usedphoneskorea.com