Look away now if you don't want to cheat the system
While browsing GTPlanet for a bit, there was an intersting thread there. It explained that you could take advantage of something in GT5 to get loads of money.
I'll quote someone from there...
birdyy on GTP said:1. Make a fake PSN account with a fake email and DOB as the current day
2. Enter in real address
3. Login, save password and make the username easy to remember.
4. Add main account to friends list
5. Switch to main account and accept fake account
6. Switch back to fake account
7. Start GT5
8. Buy used car
9. Receive ticket and gift to main account
10. Quit GT5
11. Delete the game save of your fake account.
12. Repeat steps 7, 8, 9, 10, 11 to the desired amount
13. Switch to main account, start GT5 and redeem the tickets.
You get a free gift car ticket on your birthday. If you enter a fake birthday on a fake account, you can get lots of different cars. The best way for easy money is to set the birth year as 1944. You get the Kubelwagen which is worth 73,500 when sold.