The trailer for The Vegan Revolution... With Zombies! by David Agranoff made several of my friends want to buy it. I already wanted to buy it before I saw the trailer, but it definitely reinforced the notion.
I think the key there was that it went a little viral, and a lot of people heard about the book from the trailer, who would otherwise never have ventured into the unhallowed lands of the Bizarro scene to find out about it any other way. Some of them bought the book as a result.

Trailers translate into sales principally where the trailer increases the potential market base reached. They have the secondary effect of reinforcing potential sales where the audience overlaps with other media streams, which is not at all a waste of time and money, but it means very little in terms of sales.

Yes, I've done marketing and promotions, can you tell? :p