I'll be in Changsa South station on Jan 31st. I'm going to fenghuang by bus via Lituo Bus Station.

I don't think it is necessary to book the ticket. You can just walk to the coach station after getting off the train, and get a ticket on the spot. By the way, the schedules are 12:30, 15:00 and 16:30. If you still wish to book the ticket, try to search the website by "lc96228" and it only has Chinese version.