I like the controversy in this topic, but I guess I'll second a statement already said.
If you do not use the contents in your purchased DVD or Book, then you wasted your money. If you sell that to a friend/fellow-magician, for equal to or less money than you originally bought it for, then the creator still got paid for that item that is only being used by one person, and you experience no-to-little financial loss.
Sure; You know the secrets now, I just hope your ethics prevent you from telling others the secrets (beyond the person you sold your item to).
Now if you use the contents on said item, that makes it a little different of a story, with a few variants. In that case, I think it depends on the type of DVD you're selling.
Is it a beginners guide? (Like Bobo's Modern Coin magic, currently on DVD format as well) If it is, it seems a little more ethical, because you're trying to help somebody else get started.
Is it a collaboration of tricks? (Like Eric Jones' "An Extension of Me") In this case, I think it depends on how many tricks you perform here. If you use a very small amount, it's similar to the wasting your money concept. However, if you use a good portion of the material, then I believe you're scamming the creator, because on a $30 DVD, they just get a fraction of what they should've from the DVD, making life as a creative magician harder, and can discourage future products.
Finally, if the DVD/Book is a one trick pony (Like "Triumph" on this website, or David Stone's "Window", or "The Manchurian Approach")? In this case, I'd say it is very unethical to sell the DVD/Book, because, as stated before, the original creator is only getting half the income they should, which might not sound like much, but every DVD scammed that way is preventing the creator from getting $30 or whatever the price is. And that money would go to their house bills, their food, or into making new tricks for us.
Sorry for the Length, and thank you for your time,