What you want to stock for the store to begin with would be the fast moving parts. I wouldn't start stocking expensive, slow movers like control panels, ERC's, etc. Even though I live in an area where we have 3 major parts distributors, I still sell parts on saturdays and sundays when the big stores aren't open. If your in a rural area, your not gonna have the volume of stores in my area so you have to stock accordingly. There is no set rule of thumb, You'll have to wing it. You don't need to stock every part, no one can.