1. Head to your house or any Horizon Festival
2. Go to the "Auction House" next to the "Autoshow"
3. Click on "Start Auction", select the car you wanna sell and set the price + how long you want the auction to run
4. Start the auction and wait for it to complete
If your car sells, you'll get a notification + the credits. If it doesn't, you can collect it anytime and then either put it back in the auction or gift it off to some random fella