Do bus for Guilin leave from Dazai or from the "parking lot" near Dazai ???

As I know, from Guilin Railway Station, there are several buses to Dazhai and you may need to book in advance. Many hostels in Dazhai can help you to book it in advance for you. These buses take around 2.5 to 3 hours to reach Dazhai Park Lot.

Or you have to take the bus at Guilin Bus Station to Longsheng County, then transfer to local buses to Dazhai, which will get you to the scenic area.