Yes in order to buy or sell a car to CarMax you'll need to schedule an appointment with CarMax so they can appraise the value of your current car.
You can Schedule an appointment for CarMax through their online website Carmax.com or call CarMax at their phone number which is (800) 925-3612
CarMax will buy your car from you even if you don't buy a car from them as long as it has enough value to them.