MUST HAVE THE FUNDS TO AFFORD THE VEHICLE
MAKE SURE YOUR LAST LOCATION IS YOUR APARTMENT..
Have a full garage,
Go on the car you want to buy on the internet on the game,
Press order.
It will tell you to replace a car from your garage,
Hover over the car you want to replace with your new car.
Whilst hovering over the car,
Press pause and slide to XBL STORE.
Press A, when it starts to put you on the store, spam the hell out of the A button,
Then it puts you in the store,
Spam B to get out of the store,
You should spawn in your house with all your money.
Go to your garage and your new car should be there.

Start with a very cheap car, just to reassure yourself it works.