Legal questions about book loss?

Questioner

Suppose I buy a book at a thrift store. I receive a receipt. However, the book should never have been returned there, because it is the property of a library. I discover this and contact the library, which then contacts the borrower. Am I obliged to return the book (if so, to whom? The thrift store, the borrower or the library? Do I get my money back? Can I claim a finder's fee?) or is this a matter between the library and the borrower that I have nothing to do with? Does it make any difference what the value of the book is and whether it is a book or something else?

Lawyer

You can return the book to the library and then ask for your money back at the thrift store. There is no finder's fee in these cases. You did not find the book, you bought it. The value of the book is not important.

Questioner

Can or MUST I return the book?