Seems that many chess books are out of print. For example I have been trying to get a copy of Grandmaster Repertoire 1.d4 Volume 2 (Amazon) but many sites (Barnes & Noble, Quality Chess, ChessHouse, etc.) list the book as Out of Print, Sold out, etc. Most individual sellers (e.g. on Amazon or Ebay) sell the book for ridiculous amounts (100 dollars or so). What is the best way to get out-of-print chess books like this?
You would need to look into other sources such as newinchess.com (https://www.newinchess.com/Grandmaster_Repertoire_2____1_d4_Volume_2-p-1974.html?&Currency=Euro)
However, that is something that happens with all other niches. Like comic books, once it is out of print, it becomes very expensive to get.
Unless you really want a particular book, I actually do not recommend online used book stores. They typically realize that a chess book is out-of-print, and price it accordingly...meaning high.
The single best place to find an out-of-print chess book is eBay. Most books, unless INCREDIBLY rare, get sold here at least periodically.
Login to your account, and search for the book you are looking for. Get a sense for how much it goes for (search completed and sold listings too for a more accurate estimate). Then save your search (this is why you need to login). The saved search will now email you every time a new one becomes available. If the condition and price are to your liking, buy it.
I have a book that I want to get a copy of, and while somewhat hard-to-find, I still get new listings every few days. I have passed so far because some are listings by online used book stores, and they price it high as I mentioned above, and a few, I passed on because I am a collector, and am willing to wait for an excellent specimen. I do not just want "good". Most of these book sellers want close to $100, but one recently sold for about $30. They can be found.