I have no problem with the quality of used books, when they are what they say they are. I buy anything old or unusual on AbeBooks for the most part, which apparently is part? owned by Amazon. There are some POD results, but they seem to be mostly correctly labeled and to not flood the marketplace nearly as much. I own a lot of books that were printed in the 1800s but still in great shape.
This phenomenon is definitely most prominent in the market we're talking about (though I've also seen it with much newer books), so to take an example, consider Carlyle's History of Frederick the Great.
https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Da...
The first print copy in my search results is titled "History Of Friedrich II. of Prussia - Frederick The Great-Complete Table of Contents: 22 Volumes". This is literally a print-on-demand copy of the table of contents. The first reviewer also notes that it's "hard to read." It also costs $10. The next print edition is also a POD of the table of contents. The next is a print edition of the "first volume" (what the first volume actually consists of is not mentioned; the work was published in different editions with different volume counts) by Forgotten Books, which is a relatively OK POD publisher. (I would buy it if truly desperate, but not normally.)
Continuing to go through this search, I can't confidently find a single good copy of any volume. I think maybe the "Elibron Classics" copy of Volume X is good. Some of the POD are better than others. Some of them are clear scams - for example, the "Works of Thomas Carlyle" for $14.40 that claims to be his complete works...in 336 pages. (Carlyle's actual complete works would fill quite a bit more space.) I think the $1500 one on page 2 might be good, but there's no picture.
It's not that good, cheap copies of the book don't exist. I recently bought a full set of an 1900 edition for $80, which was still higher than I liked, but finding full used sets can be difficult. Here's tons of copies of "real" volumes:
https://www.abebooks.com/servlet/SearchResults?an=Carlyle&so...
This is an especially egregious example, since I don't think the book has been reputably printed since 1969 and it's in the public domain; for most books the bad stuff is merely interleaved, instead of being almost all you can find.