What I look for are books of special interest, to a small group of people, that are probably not mass produced, not 100,000s made.
I recently sold another book, Airplane Construction and Repair, copyright 1931, 4th impression for $40.00, which included the shipping. I paid less than $3.00 for the book. This is another book that a small group of people would be interested in. And even though many of this book were made, there are not a whole lot of good copies around anymore.
Here is another example, books that almost have a cult following: Doc Savage books. I sold 7 of these paperback books for $28.00, and I paid $3.15 for them.