@Pelegrin, I didn't know that. Thank you!
So
buenos is not used as an informal greeting, only
buenas; or obviously
buenos días /
buenas trades /
buenas noches! Why don't they teach us this in all these language courses? It was only when we were in Spain that we noticed that people simply said
Buenas! when entering a shop or bar.
You are of course absolutely correct, even the RAE knows this
:
Y para cualquier hora existe, además, la expresión abreviada buenas, que solo se emplea en algunas zonas ―en España, en el área rioplatense y en parte de las áreas andina y centroamericana― y siempre en registros coloquiales.
I had to look up "rioplatense".