Every now and then I run ads on Craigslist telling people that I buy gift cards for 80% of the amount on the card. I only buy certain cards though. Home Depot, Lowes, Wal-Mart, Sears, Grocery Stores & Harbor Freight. I don't shop at any other stores and these are the easiest to resell.
Here is a tip: If you buy gift cards from people off Craigslist, make sure you meet them at the store and spend the entire card on something before you pay them. This way you know the funds are good. If you just verify the card and stick it in your wallet for later use, the seller can call the store with the receipt, cancel the gift card and get another one mailed to them, leaving you stuck out.
During the Christmas holidays on eBay, these gift cards were selling for 98-99% or even full price so people could avoid a trip to the store and have them shipped to their relatives for free.
If you're ever making a large purchase, for example a ride mower, it may be worth it to buy these cards off of eBay. If you can save 10% off a $1500 purchase, that's some serious dough.