Where to buy a second-hand smartphone in Seoul?

I'm currently travelling in South Korea, home of major smartphone companies such as Samsung. It's a country in a technology boom where everybody wants to have the latest gadgets.

That implies they no longer want their gadget from last year or two years ago. So I'm hoping I can buy a used smartphone at a bargain price. But so far I can't find any for sale!

• There's a huge electronics market called Yongsan Electronics Market, but it's not very like its counterpart in Tokyo, Akihabara - I wasn't able to find any shops selling used items.
• Though pawnbrokers do exist, they are very hard to find, work very differently to in western countries, and don't operate like shops/stores. I went to one with a Korean friend but he had trouble understanding what I was expecting to find and the proprieter at the window said they had no phones anyway.

New phone shops are everywhere, phone accessory shops and roadside stalls are everywhere. Everybody I know, both local and expat has a very modern smartphone but doesn't seem to be able to answer my question about what happens to peoples' previous phones when they upgrade.

I believe most phones here are unlocked and people buy their own phone rather than get it with a contract as some kind of rent/lease package. That makes me believe people buy and own their phones ... but I haven't been able to find out for sure.

Is there a way to find and purchase a used smartphone in Seoul, South Korea?