The Avanza bus takes the long way via the coast and up the winding road. If you want the more direct route look at this, however, the web will not update more than about one week ahead. So look at the same day of week during next few days and you can see the times. They should not alter. This service used to be run by Los Amarillos until early this month and has now been taken on by DAMAS who apparently will offer a better service. The bus goes inland up the Guadalhorce valley and then comes into Ronda from the northeast. It's the one I would take.
Hello- Is the Demas bus leaves from Estación de Autobuses de Málaga? Also, could I purchase the tickets the day of departure (I would like to take the 8AM bus) or is it recommended to purchase a few days before?
If you go on the day a little earlier you can buy a ticket. If it is a matter of a very important fixed timetable and you cannot afford to miss taking that bus then get a ticket before the morning just in case.