Some clever hackers have been able to crack the algorithm Apple uses for their gift cards allowing them to create gift certificate generators. All the sellers are actually selling are the the code which are sent directly to the customer who then inputs it into their iTunes account, and some of these sellers are selling $200 gift cards for as low as $2.60.
According to the owner of one of the sellers the business started about 6 months ago, Apple hasn’t commented on the issue but I wouldn’t be surprised if they changed the algorithm for their gift cards rather quickly (especially since it has been receiving a lot of press over the past few days).
What confuses me about this is why anyone would purchase these counterfeit codes anyway when bittorrent is even cheaper and is less likely to be cracked down on. That is unless you are using the purchased code to buy apps in the App Store.