Right all of you who are stuck doing the car sell glitch, let me help you.
Go into an invite only session, make sure all PV's are in your garage.
Get in any car you like, then ring Simeon (can ring Ron or any other character that makes a pop-up screen appear)
Drive to the garage doors at Weazel Plaza and push against them with the car, MAKE SURE YOU LEAVE A TINY GAP SO THE GARAGE DOOR CAN OPEN A TINY BIT BUT NOT FULLY, get out the car then hang up the phone.
Drive out the car you want to sell, then when you spawn outside, wait for a little yellow circle to spin in the bottom right (if it dont appear, try again)
Once thats happened, crash on the way to cause some damage to the car, drive to the LSC doors till they open and put the 2 front wheels in, with the back of the car out of LSC, call the mechanic and drive in to LSC, order ANY car (you dont need to buy a rebel or anything), then press right on the dpad, click sell, then click the value, then wait till the little orange circle appears again, once it's gone, sell the car.
Go outside and get in the PV you ordered, call MMI and get the original car back, then drive your PV into your garage, keeping your original car and keeping the car you ordered. Then quit your session and rejoin an invite only again, and repeat.
This method works 100% with me at the moment. (on PS3)
You do not need to purchase any vehicle online whilst doing this, and you still get the full price for selling it.