Get the 20rd Magpul mags that are blocked to 7 rounds - I hate my 5rd mags because they leave nothing to grab onto should something go wrong (I've yet to have a problem, but just in case).
I would sweep up magpul ranger plates and their glock magazine speedplates... Only one i know that carries the speedplates is outdoor america and thats if you can catch them in stock
Lower parts kits without the pistol grip, trigger guard, trigger & hammer. These always get tossed into the junk bin when I do a build because I use upgraded parts. LPK's with MOE trigger guard & pistol grip would be a plus as well.
P-Mags if the price is right. I need some Foliage Green windowed 30 rounders right now.
National Match front sight posts would be nice as well. I try to upgrade all mine, but I'm short a couple right now.
100 count packs of 6" wooden Q-Tips. Other specific AR cleaning items such as chamber brushes.
The one cheap thing I'd love to see is an oiler bottle that would fit perfectly inside the MOE/MIAD grip. It's the only thing I carry in there and the ones I've found are a little on the small side.
+1 on glove size trigger guards.
carry some tools too, lots of people stop in looking for ar tools and I do not carry them because I don't sell black guns.
How about armorer's wrenches? Lots of people buy a stripped lower, LPK and stock kit, only to realize they don't have the proper tool to tighten down the castle nut.
I have not built or done much on the civilian side with ar's but will offer what advice I can. I like the carry case idea. If you are looking for smaller (cheaper to stock items) lubricant, cleaning kit items such as bore and chamber brushes. Slings anything from hunting slings to tactical. The 6in q-tip idea is good too. One thing I have not seen is zeroing targets or just targets in general. And what every gun store needs a container of P-38 can openers
I know this is kind of random but I think a large selection of popular items such as Magpul stocks, not 100 of each, but one or two of each one, maybe one of each color in the most popular style.