http://blackstar-auto.webs.com/
I'm working on a car trader site. Figured once they release Share Cash and GTA Dollar micro-transactions, it could be a big thing. Just have to figure out how it would make sense and how to remove the tracker from your own car/if that is even possible (think it is 'coz I did it once during the tutorial somehow).
The site's still WIP but I invested a total of like, 15-20 minutes in there so if it's no way possible - no loss at all. I already have read or heard people talking about wanting to trade their cars though so I figured I'd jump in while no one else has done this yet.