These days, it is hard or nearly impossible to live a day without mobile phones. We use it to contact with friends, family, or someone else. Some people use phones as a music player, GPS, gaming machine, and so on. Moreover, as smartphones are becoming more popular than ever before, the second hand mobile phones’ demand has been also increased. To buy a new mobile phone is the easiest way to get one. However in Korea, the 2 years contract is essential if you get a new phone, so it is not easy to get a new one for foreign students Of course, there is non-contract unused smartphones, but they cost so much money, so I do not think that it is a good idea to buy a new phone with no contract. So today, I am trying to tell you about how to buy second hand mobile phones in Korea.
Source: http://www.nigerianbulletin.com/threads/5-things-to-do-to-make-your-smartphone-last-long.120048/
The private transaction is the cheapest way to get a mobile phone, but there is a problem. There are so many frauds in private transactions. So I do not recommend to foreign people to buy a used phone by this, but if you need a mobile phone urgent, I recommend to you to do face to face trade. Face to face trade has the lowest possibility to be rooked. So what else can you do?
In Korea, there are a lot of open online markets, such as Gmarket, 11st, Interpark, Auction, and so on. In the past, they did not deal with used phones, but as the demand for smartphones in Korea is increasing, they started to handle them. Moreover, they also provide English sites as well, so it became really easy to buy used phone to foreign people living in Korea. G market provides the best English site out of other markets, so if you are interested in buying a second hand mobile phone, it will be a good choice.
After you get a used phone, you have to register that phone. To register a mobile phone, you must be at least 18 years old. You have to bring your alien registration card and passport. Also you have to show your credit or debit card and have a Korean guarantor. If you do not have one, you will be required to pay a deposit that can be as high as \200,000 or more.
Source: https://bnbheroblog.com/2014/12/18/unlimited-high-speed-data-usage-local-sim-card-rental-korea/
For most of foreign people in Korea, it is really hard to register a mobile phone. Most foreign people are only allowed to register for pre-paid card options. Moreover, since pre-paid cell phones are not allowed to trace, some mobile companies require the account holder to return to the provider every 3 months to confirm his or her identity. Some companies have also charged some foreign account holders a \10,000 fee each time, but you do not have to pay this re-registration fee, so watch out for that. If the provider insists, call 120 (Seoul) or 1330 (anywhere in Korea).
If you just stay in Korea for a short time, you can rent a phone. The major mobile phone companies like SKT, KT, LG, they have rental booths at Incheon International Airport. So if you do not want to buy a phone, you can just rent one. But you also have contract with the rental service, which you cannot break the contract before 3 months, so you have to be careful for that.
Article by Junhyung, a Freshman in Business Administration
myHUBS 
thanks for sharing this article its very useful to purchase the second hand mobile phones.