If I had any clue my codes would be worth that much money on ebay I absolutely never would've used them. Damn I had no idea they were worth so much to some people.
I saw a guy trying to sell a "Ferrari F10 DLC code" for like $70. I'm thinking WTF, are people actually advertising as DLC codes for what they intend to send you as a gift and getting away with mad money? If so, sounds like a great freakin way to make a few extra bucks while playing the best game of all time!
EDIT: Ahh, this is drivin me nuts! I go type in GT5 DLC cars in ebay main page and first result is someone selling the Jaguar Chromeline (free from facebook) for $16.50 with 4 bids already. Maybe I should stop goin crazy and just go get some of those codes and put them up for bid myself. CIAO!