The website Music Magpie* allows you to type in details of your CDs & DVDs, and it’ll give you an instant cash price, though you need to trade in a minimum of 5 at a time (and up to 500!). To do it you just type in the barcode numbers and follow the on-screen instructions. The site also buys computer games, but for the most part you'll do better by trading them in at high st. game shops.
If you accept its offer, it’ll send freepost stickers for posting and you’ll get cash upon receipt. Someone with loads to get rid of could speedily make money this way.
How much? It depends on the CD of course, most CDs will get you 60p-70p though some go for over £1. DVDs go up to £3.
Can’t I do better elsewhere? Yes, for more modern CDs/DVDs you’re far better off selling them individually on eBay* and other auction sites to get much more. Where MusicMagpie wins out is on convenience, not price.