how do i buy and sell vehicles?

If you are in the city view, press sell or buy next to your vehicle picture.

Are there different places to buy different vehicles? I noticed that the yellow mustang isn't available to buy in most of the eastern hemesphere.

The vehicles you can buy are depending on the tech level of the city.
So if the Mustang ist Tech Level 5 you cant buy it in cities that have a lower tech level.