Well, I've seen a lot of confusion within the forums, with different methods and people mentioning that it isn't working for them.
So I'm just going to quickly write a text tutorial, of the way I've been successfully doing it for the best few days.

1. So firstly, you're going to drive your 50K+ vehicle to LSC, and park it outside.

2. Once parked, go and find a random vehicle, which sells for 50K or lower, and drive it into the LSC shop,

3. Once inside, quickly press start, then LB and double tap A, this should send you to the Xbox Store.

4. When the store has loaded, press B, it should send you back into the LSC workshop, NOT in your personal vehicle, you should still be in the vehicle which sells for under 50K.

5. Go down to sell, and click sell ONCE, do not sell the car, and then go back to the Xbox store by pressing Start, then LB, and double tap A.

6. Once again, the store should load, click B and this time you will spawn in your personal vehicle which sells for 50K+

7. Drive your car into the LSC shop and it should appear with 'Sell vehicle' and the price, which will be more Han 50K.

8. Click A twice to sell your vehicle, once the money appears into your account, press Startc go to the store and double tap A. When the store has loaded, press B repeat.


If you have any questions, just drop a comment and I'll be happy to help you guys!