Use code BOMBUR to get 40% off nearly any of their used items (when you buy 4 or more). You can mix and match the media - it doesn't have to be 4 DVDs or 4 books.
I frequently purchase used DVDs, Blu-Rays, and books from these guys. They're pretty decent. Shipping isn't free, but it's pretty cheap. In my experience, they're pretty decent about shipping things out quickly too. I often get stuff from them in 2-3 shipping days at the cheapest shipping option.