It is located in the north-west of Los Santos near the Clothes shop. Tor buy a car you need to take the reqiured ammount of money with you before you enter the trade center.
To buy a car from other players you just need to select one of the cars in the list and press the buy button. Make sure you have no car outside of your garage. Also remind to take enough money with you.
To sell a car select the second tab. Your current car which is outside of the garage will be selled. Remember how much this car costs at the normal car shop and set the price fair. Once selled the car will dissapear so you can buy a new or get one from your garage.