They were placeholders for future usable car dealers in case Eden/Atari released more DLC cars and needed new buildings to store them. That didn't really happen though.
Only one Megapack was released for PC in 2008, so some of these O'ahu Cars fake dealers turned into real ones, for example, the one in the photo below. Since you can see the 'default car' behind one, it would indicate that you haven't purchased/installed the megapack yet (or there was a server glitch at the time or you've been swapping files). The default car is shown when you don't have the actual car downloaded. Based on that photo, I think it represents a Dodge RT Challenger.
This is now an actual usable dealer where the Holden Efijy concept car can be purchased in game:
This is a shot of a Dodge parked behind the classic car dealer. You can tell it's not one of us since there's no driver in it.