In order to sell cars in Rocket League, you will need to have a valid trade key and access to the game's trading menu. To find the trading menu, simply press the Trade button on the main menu. From there, you will be able to select which car you would like to put up for sale. Once you have selected a car, you will need to set a price for it using in-game credits. After your car has been listed for sale, other players will be able to browse and purchase it through the same menu.