Gran Turismo 3 - Proffesional Mode

Easy Money:
You need two memory cards to do this. After you've earnt about 200,000 credits, buy a good car car and upgrade it to the max. this will leave you almost broke, so save this car to the memory card in slot 2. Now return to the title screen and load your last game from slot 1. This will give you your money back and you can now go and trade the car from slot 2. The car that you just upgraded to the max will now only cost retail price.