honestly don't think it works anymore. Roockstar is cracking down on any new glitches. Just wait until a new one comes out

You may need to sign out then sign back in, I was doing it and that fixed it for me.

I was doing it all day today so what you have to do is go into an invite only session then park your car in front of LSC and make your spawn location last place. Once you've done that go get a random car and take it into LSC. Click sell once but do not confirm it then hit start go to leave GTA online. Once the story mode loads, save it so you don't loose your progress then hit start and go over to game and click on start new story mode. Once the new story mode loads go to start and go to invite only session. Once in the invite only session it should spawn you by your car and by LSC. Take your car in front of LSC then get out and go get a random car and go into LSC. After that click sell once again don't confirm it then hit start LB, A, A, B then it will spawn u in LSC with the random car. Then hit start, LB, A, A, B then it should spawn you in your personal car in front of LSC. Then take it into LSC and you should be able to sell it. Click sell then start, LB, A, A,B and then it should spawn you in LSC in your car again and sell will pop up again then grom there just keep doing it.