Cardcash.com is a website where you can buy and sell gift cards, some times at big discounts! I always used my gift cards, so I've never had a desire to sell them, but for many they hold on to these knowing that they will NEVER USE THEM! If that's the case, why not sell them so that somebody who wants it can use it?
As for buying them, there are some serious discounts to be had! Here's a list of some discounts for major stores:
Ann Taylor 27.5%
Dining dough 51%
Five Guys 12%
Girogio Armani 13%
Lane Bryant 25%
There are hundreds of these gift cards with many not shown. Here's the important thing to remember though. The discounts on these cards operate on the same principles of supply and demand that all of capitalism does. So the gift cards where there is a high demand such as general merchandise, airlines, gas, and grocery, there will be a small discount compared to the speciality stores.
Regardless though, you should never buy one because there is a big discount or you think you might use it one day. That'll end up costing you wasted money in the future. Only buy these if you know you're going to use them for a specific purpose.
For instance you know you're going to be spending $100 at Home Depot next weekend. Not, "I know I could spend $100 on something at Home Depot."