Also, a quick reminder to be careful whom you kill. There are some traders that may or may not have combat options pending on your actions. And, it sucks to find out you killed the only Witchcraft skill-giver in the area... hypothetically

And, some NPCs will trade before combat (ALWAYS hit the trade button before exhausting dialogue with everyone). Some NPCs will drop all items they sell/carry, while some will not. There may be a list of who does what, but without resorting to spoilers and tons of research, I'd say buy before you kill as a general rule.