you can sell cars full price long process.
1. purchase karin rebel wait for it to be delivered to your garage
2. drive rebel to ls customs pull in barely over front tires
3. call mechanic when list is pulled up drive fully into ls customs then let your friend take over the rebels drivers seat. after you leave ls customd vack out of the phon call and close the list
4. go back to your garage get your super car drive to ls customs and repeat same process with it. with the mechanic list pulled up, park supercar next to rebel
5. walk to rebel and pull your friend out then get back out of the rebel using the triangle button
6. Have friend run out of garage and your personal vehicle should be there
7. have your friend get in your newly spawned personal vehicle and drive it away far enough so you can finish with your mechanic
8. while you are still on mechanic screen now you want to go to the rebel and confirm delivery of the rebel by pushing the X button and then press right on dpad this will enter u into los santos customs. go to sell you see the full price available for your car
9. Once you have sold the car, you should have the karin rebel outside drive it back to your garage. Do not enter garage. Get out and walk in garage then come back out drive the rebel
yeah I know this glitches kind of long-winded but its a glitch, it will get You a ton of money