You could sell it on eBay, or on the Firm Ya-Ya Swapmeet. Those would probably be the best ways to sell it. If you've never done eBay before, it's pretty simple to set up an account and list your item. They have excellent tutorials and instructions to take you through the whole process.
You can also sell it on Amazon.com -- in the sports section. I have noticed they have most of the Hardcore series, and there are several of the workouts being sold used or 'like new.'