I must say I have the same problem. I'm only testing my cars (have them all) currently, and doing one race each (plus practice runs on 4 tracks). I still get so much money with it that I constantly have to buy the most expensive cars so the money doesn't go to waste. I'm sending 1 car per day to my second account when I have duplicates that I replaced with lower mileage versions that came up in the OCD. So now I have something like 1200 cars in my garage, and am seriously thinking about just deleting some, because you can't sell quite a number of them and they just clog the garage. What am I saying? Selling them would get me more money, which I don't really need.
Then again, writing this, I realise I could just let my account stay topped up at 20,000,000 and not buy anything. That way I'll always have the necessary dough to buy what I want and I won't have to rent extra storage space for my garage.