There was an article on Slashdot a few months ago about bogus $200 gift cards being able to be generated that worked, but were completely fraudulent. I am sure that they have long been since patched and any iTunes accounts that used them banned.
My take: There are people selling gift cards for over the purchase price. This is normal, because people buy them and use the serial number for some reason, perhaps Apple doesn't take the credit cards, or they are sending a gift to someone remotely and E-mailing them the serial number.
Under the purchase price by any real margin gets me concerned, mainly because there is that suspicion that they were purchased with hacked credit cards. With all the blackhats attacking Apple, it isn't surprising to see a lot of hacked cards for sale.