Baz,
It's a real shame that people let you down especially that you are providing spares that are sometimes very hard to find or would otherwise cost a lot of money.
The answer is simple you are respected enough on here to be a 'trusted' source so people should be willing to part with their cash up front.
So don't waste your time and effort being a debt collector and focus your efforts on people that trurly value your time and efforts.