There was a thief of famous old books from the monastery Mont Sainte-Odile in France / Alsace. As they found out after some time (riddled as he broke in several times without them noticing) the the thief entered through secret stairways and chambers. [1] If I remember correctly the thief didn't try to sell them but hoarded them.

There are many other articles about the heist if you google for "Book Thief Mont Sainte-Odile".

[1] https://www.goodnightandgodbless.com/travelogues/the-book-th...