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
I'm not, I'm saying the bar is very high.
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 $5 resale value for a book it's chicken feed, barely worth considering in the value equation.
What was the name of the book?
Not OP but a cursory search would suggest that it is this book:
https://books.google.com/books/about/Statistics.html?id=wj3C...
A decade ago, I bought a used textbook on statistics, published in 1998. Highly recommended by many, written by a UCB Statistics professor (now deceased).
Total Book Price: US$ 3.94 Shipping: US$ 3.00
For just a smidge over $5 dollars, I got a world class textbook written by one of the best educators on the planet. In the era of coronavirus, I've finally sat down every night with it and have put in about 40 hours reading it (mostly doing the exercises), checking out some of the citations and references, making Anki cards, etc.
As best I can tell, creators get paid by putting in high quality work of relevance and lasting value. Your average Youtuber ain't it.