Here you go on 25 easy steps:
I did this yesterday to buy a car which I didn't have enough credits for, so I sold all possible cars, bought my very expensive car and did the trick.
1. Get a USB thumb drive.
2. Insert USB into PS3.
3. Turn on PS3.
4. Create new PS3 user.
5. On the new user create a new PSN account.
6. Start GT5 on original user.
7. Go to Game Saves on original user.
8. Copy GT5 save to USB.
10. Start GT5 on original user.
11. Buy an expensive car or use one you already have.
12. Select "Give to Friend" under car options, give the car to a friend or to new user.
13. Exit GT5 on original user.
14. Got to new user on PS3.
15. Start GT5 on new user.
16. You will receive a message that you have a gift.
17. Open gift or keep as "item".
18. Give the car back to original user or friend.
19. Exit GT5 on new user.
20. Go to original user.
21. Go to Game Saves.
22. Select USB under Game Saves.
23. Select GT5 file on USB.
24. Copy GT5 file on USB by pressing triangle then copy.
25. You have your old game save again with your money and cars.
