eBay, Amazon, Play.com
If they are Hollywood blockbuster type films you will get around £1-50 for them or up to a fiver for the latest (ish) releases.
Why not do a search on eBay for a couple that you have. Tick the 'completed' box and you will see if they sold and for how much. That will give you a good idea of how much to sell them for.
Try eBay or Amazon. Those are the two most popular sites for selling your items. I can't guarantee it'll be fast, but a buyer will come along. Many people come to these sites to look around, shop, or who are on the hunt for a specific item(s). Good luck and happy selling! Hope I helped.
Used dvd's seem to command a pretty high price on Amazon
Craigslist....take a picture and list them all works well
I have about 40 old dvds i want to sell but want to get more then 10p each for them.Does anyone know a good website to sell them to so i can get money fast?