Where can I purchase the Marozzi bus ticket in Sorrento?
According to RS book, 4euro surcharge purchasing on board bus. Is online the only other way?
Try one of the travel agency shops in town. The other Sorrento to Rome bus service Curreri
also requires booking but no upfront payment. You are emailed the reservation receipt.
haven't tried recently, but I think Marozzi sells tickets online:
http://www.marozzivt.it/