You can use buonasera to greet from 2 p.m. The literary translation is good evening, but it includes in it the sense of "afternoon" . (altough "buon pomeriggio" exists) Buonanotte is meant to wish a good night, when you are supposed to being about to sleep. *" buona serata" is often used, too. Especially for someone that is supposed to have different night. For example, if your sister is going out to the cinema/pub/restaurant/date, you tell her "buona serata"

"Buonasetta" doesn't exist ......

We use buonasera after 12 pm and buonanotte only to wish somebody goodnight, anytime from 9 pm on

I presume that Buonasera most used when you meet someone after 16:00 and Buonanotte serves to wish somebody Goodnight around midnight:)