I would hazard a guess, on items like springs that he has someone else make for him, that he waits until he gets enough orders to do a bulk purchase (price break or minimum order) from his spring maker.

The reasonable thing to do would be to take pre-orders and inform purchasers about any order requirements on his end. Asking for a deposit wouldn't be unreasonable under those circumstances.

I'd love to get the progressive rate springs for both of my cars, but there's no way I'd order them unless he actually had them in stock, given his past history.