I posted seven DVDs for sale on Amazon.com Marketplace last night, and by this evening four of them had sold already, and for decent prices, too. Selling most books doesn't seem worthwhile considering the heavy price competition online, but DVDs retain their value like BMWs, and old Disney or Criterion DVDs that are out of print? They appreciate like Ferraris. Never, never unwrap a DVD until the day you're ready to watch it. That's the equivalent of driving the car off of the lot; the value drops off a cliff and lands on a ledge about 30 feet (read: 10 dollars) down.
The key is to anticipate the next format change (HD DVD, perhaps?) and sell off my entire DVD collection six months prior.