What's usage of 서 in the sentence 나가서 사드세요?
I guess this is a sentence with honorific to say"Please go out and buy XX"
But what's the usuage of 서 here?

Question

Answer

~서 can be used with a "sequence"

돌려서 빼 Turn and (then) pull.
나가서 사드세요. Please go out and then buy something to eat. ('사먹다' is to eat out, as opposed to eat at home).

~서 can be also used with "cause and effect"

나는 피곤해서 지금 잘래. I'm tired, so I'll sleep now.

Answer

"서"  in that case means action order, something like "then".

Answer

"and"? 
Perhaps to connect two clauses that are following each other in time? Well, at least in imperative sentence

Verpassen Sie nicht die Gelegenheit, bequem von zu Hause aus eine Sprache zu lernen. Stöbern Sie in unserer Auswahl an erfahrenen Sprachlehrern und melden Sie sich jetzt zu Ihrer ersten Unterrichtsstunde an!