Welcome to my post
Feel free to like this if it works for you
Thanks

How To Dupe & Sell Your Car for Full Value:
Get in your Karin Rebel and take it to LSC with your friend in it
Park it like this - [ Register or Signin to view external links. ]
Tell your friend to stay in that Rebel
Get out and make your way to your garage
Get in the car you want to sell and drive it out
Drive it in to LSC and park the car next to the rebel
Get out and hold Y next to the rebel to push your friend out of the car
Get out and sit in your Adder or whatever car you want to sell
Another personal vehicle will spawn outside
Tell your friend to go and get it to drive it quite far away
Call your mechanic and request the rebel
Quickly press right on the d-pad and wait for the loading circle
Sell the car for full price

How To Take a Car into LSC:
Park your first 2 wheels in the customs
Call your mechanic
Drive in

Video: (WARNING) Squeaker
[ Register or Signin to view external links. ]