Steps:
1. Park the car you want to sell (It must be your personal vehicle) close to the los santos customs in the middle of city(the one on San Vitus Blvd)
2. Press Start, Go To Online Then Options, Change Spawn Location To 'last location'
2. Leave GTA Online and Start new singleplayer game (Won't Delete your old singleplayer campaign save)
3. After the 'Ludendorff, North Yankton, Nine Years Ago' screen press start then online, then create a invite only session.
4. You should spawn back inside your car and be close to los santos customs, park it beside los santos customs, get out and take any car thats not your personal vehicle.
5. Drive into the los santos cusoms, when your inside the los santos customs, scroll down the menu and press sell, do not sell it though, press start, go into the playstation/xbox store.
6. when your in there, back out, and you should be in your car at los santos customs, scroll down the menu and press sell, do not sell it though, press start, go into the playstation/xbox store.
7. This time it should bring you back into the car you want to sell thats outside of the los santos customs. Drive into los santos customs, sell your vehicle but right exactly after you sell your vehicle, press start, go into online, and then press swap characters.
8. Go into the same character you sold the vehicle on and you should be in your vehicle that you sold, right around the corner of the los santos customs.
9. Park your car outside of the los santos customs, steal or get into another car, go into los santos customs, scroll down the menu and press sell, do not sell it though, press start, go into the playstation/xbox store, now back out of the playstation/xbox store.
10. Now you should be in the car you want to sell thats outside of los santos customs. proceed to go into los santos customs, sell your vehicle but right exactly after you sell your vehicle, press start, go into online, and then press swap characters.
11. Repeat steps 8 to 10.