An alternative to regular on-line shops is to make use of the ever-more popular auction sites. The grand daddy of which, e-Bay is FREE to join and then members may buy or sell in one of hundreds of categories (there are typically over 2 million items for sale at any one time!)

Amazon also offer a re-sale facility for any product that they sell. In fact, searching for a product on Amazon will normally also give the best available 2nd hand price aswell as the Amazon price.