Years ago, I gave away about four hundred books to a couple of charity shops. I only kept about twenty titles. Can't say I regret it much, because I like to keep moving on to something different, and the libraries have provided me with all of my reading material since then, either for free or a 30p reservation charge (or via council tax payments, I suppose). No way I'm going back to buying books, either new or second hand.
CDs and DVDs, I keep. Must have about six hundred original DVD discs here now. Have binned many of the boxes, and just kept the discs (in a giant CD wallet) and sleeves for many of these. DVD is my format for the rest of my life.
Games, I trade in or keep.