Those are weekday times and they should be ok for seats to just get a walk up ticket. There are trains every 20 minutes to Busan on KTX, so really, you don't even have to schedule it. Same for the return.

Also, for you, if you use the foreigner page, you still have to go to a ticket window and get your tickets there.

One more tip - you can save a few bucks by getting a standing ticket. This means that you don't have an assigned seat on the train. You can sit wherever you want in any seat until someone shows you their ticket. You don't have to say or do anything. Just sit until someone shows you their ticket for you to move. Alternatively, in the vestibule of each car, there is a single seat you can fold down and sit in.