No problems, don't worry.
Pay attention to equips and cards mostly. They're usually what sell for less going to an NPC than to another player. There are also SQI ingredients and a while crap load of other things...buuuuut, you can learn about those later. xD
Just don't be like me. When I started 5 years ago, I was NPCing stuff like cards and gears...can't remember the cards, but bet I had a mini MVP in there from an OCA..