Or just gone to another bike store where the part would have been a buck or even free if you dug through the spare parts box on your own.

The guy at that shop had NO idea of what "good will" is all about.

In terms of some parts that you know will never go bad and that you know you'll use over the years it pays to go to other sources. The bearing balls we all use can be had for about $5 to $10 for a hundred depending on size and grade if you go to a bearing supply house and buy in whatever pre-packaging they offer. If it turns out that they are in bags of 500 pony up the money and then split them into "biker bundles" of 100 each and sell them to your buddy's that ride and wrench their own stuff for cost plus a couple of bucks to pay for your counting time. Or at that point just weigh the 500 and split it by weight.

You can also special order boxes of 100 zinc or nickel plated screws and nuts through fastener outlets. Again if 100 of all the common sizes is too many for you then split them up and sell off the excess in lots of 10 or 20 of each of the common sizes.

The special size of nut that you needed to fit down into the hex recess of your derailleur could have been "made" from a larger size by filing down the flats with a metal file and soemthing to hold the nut. It sounds like a lot of work but the time it takes is far less than riding to a shop to scrounge for a part.... especially when the guy says it's going to cost you $10.

If he'd told me it was going to be $10 for a nut and screw and there was no joking smile to go with it I think my jaw would have hit the floor. And then a few seconds later when the power of speech returned my first words would have been "You're kidding, right? If he launched into some tirade about taking up his time I'd have told him politely that now he's out time AND a future customer for any big items and then left. I'd never be THAT desparate for such a simple part. Especailly when he already gouged me $10 for the wheel bearing balls when there's about maybe a buck and a half max for his cost based on buying them in bulk from the same place I'm suggesting that you go and find.