The exception of buying when you get there are during the beginning or end of a long holiday weekend; special events like NYE and the day after Valpo/Viņa; peak travel periods like the beginning and end of summer travel crush times - January for Argentinians, February for Chileans, and around the middle of each of those months.
I see the Tur-Bus website supposedly now takes PayPal which may mean foreigners without a valid local ID number can now buy online in advance. Anyone want to experiment and report back?
Ida and Vuelta tickets are slightly discounted from separate one-ways and the return comes without a time or seat number which means you checkin at the counter to get a bus time and seat number. If you will be coming back during a travel crush day, you can get a bus time and seat number first thing when you arrive at your destination.