Car Exchange?
 

My Suggestion is to cash in your current car (If you have more than one, you could have a 'specific place' to park your car) then you cash it to 'get' another car for about...

$1000? Less or more, thats my suggestion, say if people want to have a new color of a car.

Basically,

Player A has Two Cars,
Player A puts CAR A into a part of the Car Dealership called "Exchange"
Player A Clicks on a new car and buys it for $1000(more or less)
Player A still has CAR B and exchanges CAR A For a new Car.

i agree with him i was thinking about this today especialy say your buying another car you should get like a trading in rate or something

Could be done I think.

Selling your cars back to the dealership for 75% would solve this problem aswell as opening doors for people who simply cant sell their car because of color etc.