From what they've said in their forums, Developer Turn 10 Studios is having trouble getting cheaters banned from the X-Box Live service because they don't have enough evidence.
Circumstantial though it may be, I ask you: Between the game's release date and now, would it even be possible to generate $2+ billion dollars in winnings? There may be the smallest possibility that someone could purchase the game and make the majority of their money through profitable buying and selling of cars in Auction House. This is perhaps the only explanation remaining other than cheating and would better explain the fact that at the time of the $2.2 billion auction closing, the winner only had a small number of the game's achievements unlocked.
Completing single player and online races are how achievements are unlocked and how money is initially earned in the game. It could be weakly argued that someone could take his brand new racing game, log in and then after completing only a handful of races, discovered the Auction House feature where the user promptly became a used car salesman tycoon. Since this possibility is the only thing preventing a ban on X-Box Live, perhaps I should give credit to Microsoft for requiring more witnesses and physical evidence than the US legal system requires to convict someone of first degree murder.
The cheat is accomplished in this simple way. A user wins, buys from a dealership or buys through Auction House any given car or cars, upgrades as desire and funds allow, then duplicates every car in the garage. This would include cars with "locked" custom designs. Design locking was a feature intended to prevent users from applying the design to other cars that the buyer could then resell.
The cheater first saves the game to a memory card then proceeds to "gift" the cars to a "fake" X-Box Live account the cheater has newly created for this purpose. This is aided by the "free trials" Microsoft offers for the gold account level that come in one and three month varieties. After the cars have been gifted, the cheater restores the original account using the memory card. Finally, the cheater may then log into the secondary account and gift back any or all of the cars that once belonged to the original account.
The cheater can then make as many duplicates as desired and sell them to the dealership or to other players via Auction House. Rather than sell immediately, though, a little more patience could generate a lot more game money. After all, you could then immediately repeat the process to make 4 of each car in the garage, then 8, 16, 32. If you've ever had to buy RAM for your computer, you know this doubling could quickly get into large numbers. At any point, the cheater can sell part or all of them to convert all of those duplicated cars into a rather large pile of game cash.
The 35th Anniversary Camaro is the one referenced in the topic. It's one of three car models that may only be purchased from Turn 10 via Auction House. The other two exclusive models are the Lotus Exige Espionage and the Mitsubishi Lancer Evolution VI TME. There are other models of Chevrolet Camaro, Lotus Exige and Mitsubishi Lancer Evolution in the game of course and they are available like all of the other cars--via the game's dealership and awarded for winning races. These specific models, however, reflect the limited supply of some special edition cars produced in the real world. Due to their rarity, players and developers have dubbed these cars "unicorns."
Since the cheaters are really only trying to preserve these rare creatures by encouraging them to multiply, maybe we should stop seeing them as poachers. In the least, they should be the hero of many a ten year old girls, wiccans and faeryfolk. Personally, I just find it hard to respect anyone who paid $2.2 billion for a Camaro that doesn't even transform into a friendly giant alien robot, even in a game.
To get more info on the cheat, search google and/or youtube for "glitch forza 360."