This is a non online glitch and will only work if you have not updated the game.

Buy any car you can afford, now buy as many parts for your car you can afford.

When low on money and you can't buy no more parts, press start and select default settings (puts your car back how it was).

Then don't go back to the menu, stay at upgrades menu.

Now sign out and back in, then go back to upgrade menu and note that you have the same amount of money you had before buying the parts and still have the parts.

Now you can sell the parts and make money!.

Go to the the main menu to save and do it as many times as you want!