This is really the key to the whole subject. Make cars realistically earn-able. Then nobody would bother to dupe.
In GT4 you could get any car you wanted within a day. You can't do that in GT5. You buy one $20 mill car and it will be about a week until you sniff 20 million.
There's 20 X1's (if you count every color)... nobody with a sane mind will earn them. This game isn't that captivating. It would take about 360 hours of grinding Indy just to purchase the X1's.
Really the whole economics of GT5 is whacked. You can't sell high end cars... you can't win enough good cars... now you can't even trade high end cars. LOL