What is the value of a $40 power supply? To the guy who built it, that may be $20 in parts and some labor, plus the time to research the necessary parts and the recognition of the need for the product in the first place. What about when he ships out the wrong power supply and then exchanges it, but through an eBay quirk the buyer winds up not paying for anything?
One of the first computers I touched with a GUI was my grandfather's Atari 520ST some 30 or-so years ago. After his retirement from the Air Force he worked in a few jobs, one as an electrical engineer designing and building motors. He used a CADD program on his ST to do work at home. "Boomers" get a bad rap today for being technologically inept and largely incapable of using computers, but not only did his generation largely invent the damned things but he and many of his cohorts did and do quite well.
He spent time with me setting up some floppies to use for drawing, my own CADD stuff, some games, even programming in GFA BASIC. While I still have those floppies, no one seems to know what happened to his collection after he passed.
Recently I picked up an Atari 520ST just like his and in near-perfect condition, though it lacks a floppy drive, power supply, and monitor. I have set out to build this system and came across a custom Atari ST power supply on eBay. But the seller shipped me an Amiga power supply! A very nice one, but I already have plenty of those. He immediately offered to exchange it. I just had to process a return and he immediately shipped me a replacement. I received a refund and several days later I received the replacement.
After a few exchanges and looking through PayPal and eBay I found I did not have a way to pay him. Not only that, but having me pay for the item outside of an eBay auction would put him in peril of violating eBay policies. Sure, there are some ways he could have handled this to square it up and get his money, but instead he offered to let me keep it.
While trying to "rebuild" my grandfathers computer -- the one which launched me into graphical environments owned by a man who had a long-lasting influence on me -- though my time-line took me through Amiga rather than Atari, I found a $40 custom Atari ST power supply and the generosity of a stranger to be invaluable.