Drive the car to Los Santos Customs
Enter the garage with the vehicle you want to sell
Select the Sell option
Confirm the sale
Note that most stolen cars can be sold only once every 48 minutes of real time
Check that the car is not too valuable, since some high-end vehicles cannot be sold
Use the car sale to get quick cash in GTA Online
Avoid selling personal vehicles if you want to keep them
Make sure the vehicle is not mission-related or otherwise unsellable