I do a lot of searching for particular books and can find some expensive titles at 5% of their normal value. I don't have duplicates that I could sell because there are always new ones I want to get.
While I do not condone raiding rare book caches, reading about James Shinn or Stephen Blumberg reminds me of how bibliomania can lead book piracy.
https://www.atlasobscura.com/articles/james-shinn-book-thief
http://articles.chicagotribune.com/1990-03-27/news/9001250309_1_rare-books-books-and-documents-priceless-books
Friends say Blumberg, 41, dressed like a homeless street person, seldom bathed and often ate in soup lines. But they also say he was a shrewd trader who roamed the nation`s antique flea markets, carried a leather pouch full of gold coins inside his pants and thousands of dollars more in cash in his pockets