About 5 years ago I went through the process of moving my books from one home to another. At that time I filled a single box full of titles I planned to sell at the local used bookstore. (For comparison I probably had 25 boxes I planned to keep.) That box is still in the closet, never having been delivered to the used bookstore.
The New York Times has a short article on the perils of 'weeding' to all of those who patronize libraries or own one themselves.