If these are going to a gunsmith a customer might be less versed in the good stuff such as triggers. A lot of folks will cheap out. I agree with Tom on the NM triggers though. I think castle nuts and sling adapters would be useful.
I think if the intention of these kits is for the small gunsmith the kits should be heavy on whats frequently lost, broken or damaged. Detent springs and pins,gas block roll pins, extractor springs etc. I like the idea of a socket head cap screw for pistol grips.
For general conversation about parts storage I use the Durham stuff. There's all sorts of variation and sizes plus they have handles. Some sizes fit in cabinets where the boxes will come out like trays. I do this at home and the office. It's not crazy expensive and it should last a life time. Just need to make sure you match the size of the cabinets and drawers.