The "Underground Car Contract" is an associate of Aiden Pearce who securely stores stolen vehicles for use at a later time. The contact also sells a select number of vehicles. According to Jordi Chin, the contact is a friend of a friend.
When a vehicle is stolen or its info is hacked off of some pedestrians, the information and details for it can be sent to the contact. The vehicle will then be subsequently added to a "library" of owned vehicles.
When vehicle registration details are being exchanged over the internet by pedestrians, the information can be obtained and then sent to the contact. The contact will then be able to swipe that car and add it to Aiden's vehicle library.