*Online, people frequently use the Naver blog Junggo Land (second-hand item land or "중고나라"). The blog has safe payment programs such as Unicro and INIP2P installed. But this site write for Korean, so it would be nice asking help from Korean friends. There is a good chance you might get scammed, so it is recommended that you use specialized intermediaries such as bizting, safeu, nescro(네스크로), unicro, inip2p to make deals with people on this blog. The five companies listed here have been operating over 5 years and are credible. You can also use mobile applications like bunjang (번개장터 or Lightning market) and givenu to exchange used goods. Other than second-hand item exchange sites, you can buy used items instead of new items at retail sites like Aladin and Interpark.
*Offline, You can also visit offline stores like the beautiful store (아름다운 가게) and sell and buy used items.
1) Seocho Flea Market(http://seoulistic.com/living-in-korea/flea-marketssecond-hand-shops-in-korea/)
When: Saturday (dependent on the weather)
Time: 9:00 – 15:00 (but try to get there between 10:00 – 13:00 in order to get the best selection!)
Where: Get off at Sadang Station of Line 2 (Green). Go out exit 12 and walk between the bakery and cafe.
Items: Clothes, shoes, accessories, bags, jewelry, watches, household items, etc.
Price range: Around 500 won to 50,000 won. Generally 3,000 – 10,000 won for most clothing. Even jackets!
Note: Remember to barter if you can! If you buy multiple items, try to haggle for a discount.
2) Platoon Seoul Flea Market( http://www.10mag.com/platoon-kunsthalle-night-flea-market/)
Where : Come out of Hakdong Station of Line 7, exit 10 and head toward Dosan Park. Platoon is a 10-minute walk away, just across from Nanumi Hospital.
Note : The Platoon Kunsthalle concept, an aesthetic compound that has its flagship in Germany, is a place of artistic experimentation and subcultural socializing. Platoon Kunsthalle hosts art for patrons who are interested in creating and displaying their work, from urban art and contemporary photography to funky music and modish dance. It is an ideal place for people to find an outlet for artistic expression. If you are interested in reserving a spot and selling your stuff at the flea market than you can sign up on the Bling website (Korean language) thebling.co.kr or contact Platoon Kunsthalle for an English sign-up form (firstname.lastname@example.org). The registration fee is W10,000.