There is a big tractor dealer that has about a dozen branchs scattered over the farm belt that stock very few parts. The farmer we lease from keeps a parts book on all his equipment and when something breaks, he calls the dealer with part #. The dealer delivers 2 times to each location daily- once at night and once during the day from a central warehouse. Most of the time he can get his part next morning or by end of business day. Of course some parts have to be ordered and could take longer. He says it works for him as long as he keeps belts, hoses, cutter blades, etc in his shop. When he sells a piece of equipment the dealer buys back unopened inventory less re-stocking. He does pay a premium for parts but it keeps his operation going.
Unless it was the wrong part to begin with, I rarely return parts. If I was operating on the scale of the farmer you refer to, I think the conversation would go something like "So, you want to charge me 15% for re-stocking. OK, since the part is now selling for 25% more than when I bought it, that means you own me the purchase price, plus 10%".
I'd sell the parts to the new owner of said equipment, or on Ebay, before I'd pay for "re-stocking".
That's the money end.... which always matters.... but what twists my tail even more is critical parts disappearing altogether - can't be had off-the-shelf for any price. Not talking about just 50 year old tractors, but rather automotive parts on less than 10 y/o vehicles.
Rgds, D.