I haven't seen anyone post about this here so I thought I would.
1) Accept the Mercedes Vision Concept as a gift or buy it. (Make sure it isn't your active car)
2) Uninstall the 1.01 patch (if you bought the game digitally, you have to uninstall completely and redownload), then decline the update after restarting the game.
3) The AMG will show up as a number in your car list worth more than 333million credits. Unfortunately the maximum sales transaction size is 20million, so sell it and get that.
4) Reinstall the patch, buy 20 more AMG Vision Concepts
5) Uninstall patch and sell them each for 20 million. You should only sell one at a time, then buy what you want, and sell the next since your in-game wallet maxes at 20,000,000CR
Just make sure to never be in the car you're selling, it will mess up your save.
The credit grind is not what I come to GranTurismo for. I like fighting against the clock more.