How can I sell a car to buy the delorian?

Go to the house where the car you want to sell is parked. Click the car. A bubble should appear above that says what it is, what it's Forbes index is, and two buttons: 'Set as the vehicle' and 'Sell'.

Click 'Sell' and confirm sale when it gives you the price. You will then lose that car (and get a minimal amount of cash back) and you can buy your Delorian.

You click on the car it's sell and then brings up a window and ask you if you want to sell