Let me start writing this by stating that SITA has become overcrowded in the last years. So, if you can take a ferry instead, that option is always preferable.
The Sorrento-Amalfi route is the busiest of them all. Salerno-Amalfi is much better.
Yes, it's safe to buy the tickets from newsletter stands, cafes, bars .etc.
A full list from where you can buy the tickets is here: https://www.sitasudtrasporti.it/agenzie (only in Italian...make sure you select 'Campania' if asked...SITA also operates routes in Puglia and Basilicata)
Check this link for what tickets you need:
Basically it depends on the departure and arrival town. For Salerno-Amalfi you need AC3 which costs € 2.40. The trip takes roughly 1 hour and 15 minutes.
If you want to reach Positano from Salerno, keep in mind that you will need to change buses in Amalfi. Amalfi is the central transport hub on the Amalfi Coast. Same thing if you travel from Sorrento anywhere past Amalfi. You need a change of buses in Amalfi.
Other things to keep in mind:
- validate your ticket once you boarded the bus
- the buses have ample luggage storage
- when you wait for the bus in a station on the road, make sure you signal to the driver to stop
- the drivers are quite helpful, so if you don't know when your stop is up (all those curving
roads will confuse you very fast), ask him nicely to announce it when
you board the bus.