The big question is: did he ask permission to each person who contributed to this? One thing is to have your words in a web site, another is to have them included in a ebook that someone else is profiting off.

However, he does not own the content or have a license to redistribute or sell it, and compiling it into an ebook to sell is clearly outside of fair use doctrine.

Did he have the permission of either Reddit or the author of those comments?

It would seem it's not necessary:

"By submitting user content to reddit, you grant us a royalty-free, perpetual, irrevocable, non-exclusive, unrestricted, worldwide license to reproduce, prepare derivative works, distribute copies, perform, or publicly display your user content in any medium and for any purpose, including commercial purposes, and to authorize others to do so. (emphasis mine)

> of either Reddit

they'd need at least.

True. After re-reading it, it's clear that I can't take content, but Reddit could authorize me to; Reddit's permission is still necessary.

The question was not "it is right to take money" but, "why would OP come up with the idea of taking money".