Which venue is best to move my unwanted dvd. Some are still sealed.

Amazon: higher fees, lower selling price, faster sale => good for time constraints
Half: lower fees, higher selling price, slower sale => good for making the most money

I've actually been surprised at what people will pay for stuff on Amazon. I've sold stuff there that has sat on Half.com for a year or more, for about the same price. I've also sold some stuff new, where the buyer paid me more than they would have paid Amazon (assuming free SSS).
I would (and do) focus on Amazon. People go there thinking "new" and are willing to pay more, whereas people who go to Half are thinking "ebay/used" and are willing to pay less. Just my experiences, and it's not 1004%, but I've moved my inventory to Amazon. The only things I list on Half are things that the Amazon market is flooded with.
The main negatory to Amazon, is SFranke says, the higher fees. They are pretty damn high. But part of what you're paying for is the Amazon name and webspace.

I almost always sell something immediately on Half, the secret is to price whatever it is a dollar below the lowest price for the quality grade. I've been shocked to see sales within an hour of posting.

The competition on amazon is something terrible. The large competitors use computerized pricing schemes making it a royal pain to sell there.

This is probably not advisable unless you spend all day in front of a PC like I do, but I have listed the same item on both sites simultaneously. As soon as it sells on one, I delete it from the other. I've only sold 9 items in my life, usually with only one at a time listed. I wouldn't want to try to keep up with a whole bunch of stuff that way. The obvious danger is that somebody will order both items while I'm asleep and I'll have to disappoint someone and take a ratings hit.