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)
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
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 so removing that artificial barrier is helpful.
How To Get Practically Infinite Money In Gran Turismo 6