I Took this from se7esins user Reflectedtaco and he found it from somebody else so I can't take credit
This will help you guys finally get you into your personal car when trying to sell it for more than 50k

1.Go to an INVITE ONLY SESSION, attempt to do the glitch (don't sell), the glitch won't work.
2. Sign out, and sign in into a NEW ACCOUNT, you will be in a heist cutscene (beginning of story mode)
3. SIGN OUT OF THE NEW ACCOUNT and back into your main. You should be at the beginning of the game (your old save was not deleted don't worry).
4. Press pause and Go to an invite only session online
5. Do glitch again, it should work (if you don't know how to do it then go to step 6+, if you do, then you are all set.

6. Park the car you want to sell near LSC
7. Take a random car and go into LSC, click on sell (don't actually sell)
8. Pause > Go to store > Exit Store. At this point you are either in your expensive vehicle or still inside LSC. If still inside LSC, just Pause/Store/Exist again and you should spawn in your expensive vehicle.
9. Once you spawn in your expensive vehicle go into LSC and sell (do sell this time) pause/store/exit.[b]