By buying used, and spending $5 for content that you've used for 20 years, I wouldn't suggest you're providing better income to that professor than YouTubers get from Ads or Patreon
Physical content like a book has a residual value; you can -- and people do--sell it when you're done. The residual value after ten years is something like 5 bucks perhaps, especially considering a new edition was published that year.
In contrast, patronage is pretty much a total writeoff.
Most people subscibe to creators to support the creator in producing content they value, not to purchase a book or something they can sell on one day. I mean really $5 resale value for a book it's chicken feed, barely worth considering in the value equation.
