The one thing this article doesn't touch on, that I wish it had, is condition. I'm a large buyer of used books from places like AbeBooks and Amazon, and the conditions simply cannot be trusted, especially from the larger sellers like Owl Books, Thriftbooks, and Better World Books.
Almost every time I buy a book listed as "Very Good" which has a clear definition on AbeBooks' website[1], I end up getting something with clear markings, or damage.
[1] https://www.abebooks.com/books/rarebooks/collecting-guide/un...