It means taken with the intent to resell, not to read.
I guess I'm happier not getting angry over things that I don't know for sure, I'm happier generally assuming the best of my neighbors, and I accept that the books are out of my control once I drop them off at the library.
Or maybe OP just means that none of them ever return; it's supposed to be a LF Library after all, not a LF bookshelf.
And there's an unspoken rule that you should probably give back, too. It's a community good, not a charity.