Their price is double the official price
I was buying tickets for KTX from Seoul to Busan. I didn't know the official Korail website so I thought buying tickets here was the same. It is not. The price is literally double (214K won vs 107K). Moreover, they sell tickets that on the official website are sold out, which makes me think they first get your money and then check the real ticket availability. I'm happy I discovered this before it was too late. Don't but train tickets here, there's no reason to pay double price.