When patrons (or library staff) come up to me and say, "I saw this book online/my friend told me/I saw in an antiques store/I heard that such and such a book is really valuable." and therefore "we should sell it!/we should never weed it/we should get someone to value it" etc.
Point 1. Ex-library books, no matter how rare/valuable/costly on Amazon lose most of their value because they are, well, ex-library books. Most of the more valuable books are older and older library books tend to have a LOT of markings. Spine label, card pocket, stamps throughout and on the edges, etc.
Point 2. Older library books, even if they haven't circulated much, are generally in only fair to poor condition. The wear and tear and being squashed on a shelf, pulled out and put back, not to mention carried home by loving (or not so loving) little hands.
Point 3. The public library is about ACCESS. I am not going to lock half my older picturebooks up in a cupboard or sell them on amazon because they have turned into collector's items. Lots of people - and kids - like older books and the bindings hold up longer than the new ones!
Point 4. I don't have time to become a book collector par excellence. If I weed something and it turns out to be rare.....that's what library sales are for! To find treasures!
I will not be locking up, weeding or not weeding, or doing ANYTHING about our Oz books, Orlando the Marmalade Cat, or Leo Politi. They're going to stay on the shelves until they're in too bad condition to circulate, then I'll find reprints and weed them!