Part of the reason I started GiftCardAdvocate.com was to educate people so that they could get more value from their gift cards.  That information includes listing all the available options for selling your unwanted gift card.  Most services listed there also sell gift cards at a discount.

I was pleased to find this very straight forward introduction to selling your gift card using one of the major gift card buying/selling sites, from MSNBC.com.  If you have unwanted gift cards but have not yet tiptoed into the resale marketplace to sell them, give the article a read.