Nitrous Oxide
To get the nitrous oxide from the used parts shop, you need to lose at the drag races on a day ending with '5' (such as 15, 105 etc). Return to the screen for the parts shop after the drag race. It will now be 16, 106 etc. There will now be a question mark (which is the nitros) at the end of all the parts, after the turbos.
Submitted by Hovey. 

Easy Money
Unlock the GT40 Concept car from the season 1 official races. You will be able to buy it for $150,000, and sell it for about $185,000.
Submitted by Hovey. 

Go to: Sega GT 2002 Info Page | nGaming Home Page.