He's not "citing sources." He's outsourcing his argument to textbooks. The point stands: if you want to refute an argument, do so yourself, possibly with reference to corroborating sources. Don't just say "you're wrong. Go read this stuff to figure out why" -- that's no way to have a discussion.

The GP never said they were wrong because they gave sources, they said they didn't give any rebuttal at all and instead only cited sources that presumably contained a rebuttal. If they said "such and such author did a survey that found that only 5% of money held by rich people got to them via productive pursuits", that would be one thing, but they just said "such and such author says you're wrong"

The point is, there’s no way to evaluate if this commenter is wrong, since nobody is going to read 7 books to verify the validity of an internet comment.

It’s a classic logical fallacy; appeal to authority. There is no reason to believe any of these writers have a better understanding of how the world works than any other “authority” of the past like Karl Marx.

Just because someone says something in a book doesn’t make it true.