Don't forget FSB parts. Most factory barrel assemblies with a pinned sight, and even my armalite clamp on come without post/detent/spring. If a customer walks into a shop to pick up $5 worth of parts...they will probably leave with more. I would also think a few gas tubes would be a nice touch. Those little things you may forget in the excitement of ordering the big build parts.
I have 3 shops near me that have small parts kits. Very handy since shipping usually costs as much as the item. I mortared a stupid buffer retainer pin the other day and couldn't find it for the life of me. They had one in the box, and I picked up a grip, rail covers, and some ammo that I had no need for.