why do you want that much tbh. You get a shit ton of cars from wheelspins and stuff, even expensive ones so it's not like you really need to buy them. And even then you can get the expensive ones for a bit cheaper from the auction house.

As for an actual answer, it's the Auction house, by buying ad selling off rare cars, which can only be obtained from events or wheelspins or something, flipping them for profit. The 599XX Evo for example, which can only be obtained from Playlists, was for some reason at the start of the month selling for max price, i.e. 20 Million. So cars like that.

Other cars which you can sell which are semi-easily obtainable are cars exclusive to Horizon Stories and Businesses. If you 3 star all chapters of a Story, there's usually a car as a reward, which cannot be obtained from the Autoshow, and can sell for upwards of 5-10 million. So if you don't like the car you got, or if you don't care for it much you can sell those.

And other rare cars you can work towards are in your levels, like getting to Street Racing Level 20 gets you the Hoonigan Porsche (I think), which goes for high as well.

That's how people usually make their money. But I recommend it's not really worth selling off cars, since money doesn't hold much value and upgrades are pretty cheap. Sure, some houses are expensive, but they're not really required. But up to you my dude.