Contributed by Coins to Notes Exchange Facilitator, Niall Ruddy
It may well be a cultural quirk but it seems that westerners tend to be slightly embarrassed about standing in a mart counting through coins to pay for something so most people will have a jar or some such receptacle full of loose change after staying in Korea for a year or more.
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From first hand experience: Go to the KEB bank branch between Sangnam/Jungangdong and get a ticket for the line from the machine to the left of the first counter you arrive at. When you are called you merely have to give them the bag/jar/piggy bank you have stored your coins in and they will dump them in a machine in their back office. Receive the money and spend it on kimchi/makoli/toy dog. You do not have to sort the money into different denominations as the machine will do this for you.
The majority of banks and the larger post offices should also be able to provide this service but some may require you to sort the coins yourself beforehand.