I've picked up some real bargins on Amazon Marketplace. I've had CDs for 1p (£1.24 P&P mind) and DVDs under £1. Bought stuff new and used on there and never had any problems. Just check the seller's feedback and description first.
However you won't be able to use vouchers and codes. To do so you'll have to leave the CDWow site, clear your history, then go back to the site in a new window (long winded I know, but its the easiest way I can find).