You'll find many charities do sort the wheat from the chaff, Molly, and a visit to your local Oxfam shop will show you just that. There's a local charity here that even has a specialist, secondhand bookshop. Business is, as they say, a dog-eat-dog world and even charity shops have to be competitive if they want to justify their existence. I'm afraid those who don't will just go under and it's no-one's fault but their own.
Just as long as your mother doesn't resort to the underhand tricks of some of our borrowers. The favourite is to ask for a fairly rare book to be borrowed via the Inter Library Loan system. They pay the fee, take the book home and then just don't return it. Three weeks later you see a similar title selling on Ebay.
Our open access policy means they don't have to provide us with proof of ID, so they often use a false address. With 20 libraries in our system alone and several staff in each one, it's not hard to imagine how often they get away with it. But it's counter-productive in the long run, because the charges get passed on to us and council tax goes up, along with the loan fees.
Apart from which it's thieving, plain and simple. At least your mother's paying the asking price for her books.