You need to rather all information in your possession showing that you sold the vehicle (i.e. the bill of sale, evidence of payment, copy of any other transfer documents), and file into the local records with the clerk of court. Additionally, you need to notify your car insurer of what has occurred, and advise them that the vehicle is no longer in your custody or control, so that if anything else happens, it is not placed on your records. Further, you need to respond to the electronic ticket and advise that you no longer own or operate the vehicle, and provide the contact information for the new owner.
The Texas Department of Motor Vehicles lists the following information at its web site concerning a seller's responsibility when selling a vehicle:
"Vehicles are required to be titled in the buyer's name within 20 working days (45 calendar days if the selling dealer is also financing your vehicle). Failing to properly transfer a vehicle into the buyer's name could result in the seller being held responsible for tickets, toll violations or they could even have property seized if the vehicle was used to commit a crime. To ensure this doesn't happen to you, accompany the buyer to your county tax office and make sure a vehicle title application is filed.
"If you sell or trade-in your vehicle to a licensed motor vehicle dealer, the dealer is not required to title the vehicle in the dealership's name. The vehicle stays in your name until it is sold to an individual, which may take months or even years. To protect yourself, file a vehicle transfer notification.
"Provide the buyer with a properly signed title, including the date of sale and odometer reading and a signed vehicle title application, with the sales price clearly shown. Keep detailed written records of any transaction, including contact information for the buyer, the date of sale and information on the vehicle, including the Vehicle Identification Number (VIN)."
The web address for that information:
Bottom line: File a vehicle transfer notification so that you can avoid liability for parking tickets, tolls, etc.