Good idea, though downside would be the advertising of interchangeability parts. Eg. most suspension parts off a R33 will go onto a R34 leading to duplicate posts in multiple sub forums.
It'l be good to categorize it down to Panels, Motors, Gearbox and other sub categories, but likewise it will still be hard to regulate duplicate posts as the current system allows people to view various ad titles for different parts rather than just categorized and having people need to go into multiple sub forums. It's not a hard thing to do, but many people will tend to not be bothered for another click or two.