Regarding trains, we used them twice – on the route from Seoul to Busan and from Busan to Gyeongju. We purchased our tickets on the letskorail.com website, where the prices were the most favorable.
There is also the koreantrain.com website, but it redirected us to RailNinja, where the prices for the same journey were significantly higher – so it's worth being careful and avoiding intermediaries.
We also traveled by long-distance buses twice – from Gyeongju to Jeonju and from Jeonju to Seoul. We booked our tickets through the bustago.or.kr website, which is mainly aimed at local users or those with a Korean phone number.
⚠️ But! If you don't have a Korean phone number, you can enter any phone number when booking.
💳 Then pay with a physical card (make sure it's the one you will have with you in Korea), because you will pick up your ticket at the bus station – either at a machine or at the ticket window. The card used for the online payment will be needed for verification and printing of the ticket before the bus departs.