If you have DVDs or Blu-Rays laying around, gathering dust, then why not make some money by getting rid of them?
The services that I have used or I have heard good reviews about are:
The above stores all have a iPhone/Android app. This will help you greatly if you have lots of items that you want to sell.
When you compile your list of items for selling, please check multiple sites.
As an example, a box-set on one site offered £2 whereas a different one offered £10.
Cam compared offers with other sites when selling to CEX to get a better price. This worked because he had taken his items into his local branch of CEX.