I recently noticed that Amazon has a trade-in option! My son had been cleaning out his media, and I thought I'd check to see if I could list them on Amazon. The items were too common and selling for pennies; however, they were offering trade-ins for no less than a dollar each. So, I printed out the pre-paid label they provided, mailed the books and DVDs, and very soon had about $6.00 deposited into my Amazon account.
To find trade-in items simply enter your item in the Amazon search box and look for the "More Buying Options" window on the right. In that box, you'll see a button labeled "Trade in Here" with a price. It is located under the "Sell Yours Here" button.
Hereis an example listing.
I used my last balance to purchase a non-Amazon gift card.