I have the habit of generating a parts list for everything I make and doing a monthly inventory...sort of. My wife does it. If left to me then it would fall apart rather quickly. The inventory sheet has:
1. The suppliers part #.
2. A thorough description.
3. The supplier name and phone
4. The quantity of the item to have on hand.
5. The quantity of the item to order.
6. The date ordered.
7. The date received.
In essence...it's matter of knowing what you need and making sure it's on an inventory sheet...and then someone has to take inventory. ...but YOU know this already.
I'll keep pickups, CTS pots, wire, jacks, switches, magnets, all the screws, drill bits....everything on hand...and I don't unwrap packages until I'm ready to install it. The parts I make also have a part number and are on the list.
I'm responsible to generate and update the inventory list....and make whatever parts I make. My wife takes inventory and orders the parts.
The only thing is...I'm still experimenting with pickups so I don't buy too far out ahead of myself with those...I do have a couple of sets of those on hand.
When I design...I do it around available hardware whenever possible...or suffer. For instance, when I designed my first aluminum pickup ring I designed it around the screw that Seymour Duncan provided with their pickups. Too bad it's a very long #2 that's otherwise unavailable. Next time I designed a pickup ring I thought FIRST of the screws that I'd have to mount it with and then designed around an institutional screw size.