Save yourself the money and stress: do not book through Rail Ninja.
I booked two First Class KTX tickets from Seoul to Jeonju, paying over $60 above official train prices. After paying, I found out our seats weren't even assigned together. There was no warning or seat selection option during checkout. When I reached out to customer support, their only solution was to downgrade us to Standard Class—while refusing to refund the price difference. Charging massive markups while offering zero flexibility or support when their system separates traveling companions is unacceptable. Do your research and book directly through official sites like Korail. Lesson learned!