It works like this... Gather up and list all your old media. Garage sales and thrift stores are a great place to start if you don't have anything at home to part with. Once you list your items you'll get one credit when it's requested by another member and you ship it out. Then you can use that credit on another item of your choice. Easy!
The site is still in beta so if you have anything you'd like to get rid consider posting it now so that the site can grow quickly and there will be more for you to request.