I have been buying machines like this and listing parts on eBay for several years now and I can tell you 100% for sure "it can't be about the money". I have figured it out several different times and once I figure all associated cost (storage rent, cost to buy machines, cost to pick up machines, fees, etc) I make less than $3 an hour doing it. But I just love saving the "junk" from landfills and getting what parts I can to people they will help.
For me it started out with machines I had purchased to fix up that were too far gone. Next thing you know I end up listing parts from those machines and my local pinball buddies started offering me "junk" they didn't have a use for at reasonable prices. 5 years later I have dozens of machines to part out (close to 100 sitting right now), people taking my "junk cabinets" etc for craft projects, parts going all over the world helping people save machines, etc.
Thankfully I am semi retired so I don't need to make a bunch of money doing it but I find it very rewarding in so many ways. To me the worst part is keeping track of everything. Be sure to mark parts with tags (or bag smaller stuff up and label the bags). You think you will remember what something was off of but after a few weeks it all starts looking the same. (Again talking from years of experience).
The "best parts" are the oddball stuff. That stuff doesn't sell as quickly but when someone needs it they are happy to find it. Chime units, bells & lock bars typically sell the quickest. Once they are sold most of the rest of the stuff takes some time. So when I buy machines I look to have the chime units, bells & lock bars pay for the machines then I just wait until the other stuff sells to see my profits.