A little late to the thread, but I thought I’d answer for future questions and reference. All knives are legal for import and domestic purchase in South Korea with some restrictions. Since 2011, knife blades beyond certain lengths are restricted and require a registration license per individual knife.
A. Fixed blade length: 15cm or longer requires registration license
B. Folding blade length: 6cm or longer requires registration license
C. Automatic blade length: 5.5cm or longer requires registration license
Anything under the above blade lengths should have no problem clearing customs. Anything above will require registration with the local police authority of the city / district you live in (if residing in South Korea) before import or domestic purchase. If passing through the Incheon airport, I believe the local authority would be the Incheon Metropolitan Police Agency. Also, if residing in Korea, international purchases through the internet or overseas dealers are prohibited from import unless you’re a domestically licensed dealer in Korea.
As previously mentioned, each individual knife with a restricted blade length will require its own registration license, as they include specific information for each knife (manufacturer and country of origin, model name and number, blade length, purpose of use, etc.).
In Korean the license is called 도검 소지허가증. I’ll link a couple of resources, but they’re all in Korean, unfortunately. Google translate can give you some idea of what you’re dealing with.
Link 1
Link