Play through Underground Mode as you would usually do, unlocking a good amount of cars and performance parts. This is just your initial investment (or 'seed money') for the trick. Save the game. Then buy and modify the VW Golf to the maximum and trade it in for the Honda S2000 for a gain of about $1500. Take the Honda to the garage and demodify it (strip the parts) and trade the Honda for a Skyline (and a loss of about $3000). With the Skyline, you can now trade for the VW Golf for a gain of about $10,000. You can repeat this car laundering until you have enough cash for your dream machine.