There is definitely some money in this game, but as Looks4u said, you need a decent knowledge of cars and what to look for. Being able to repair and service them is also a big bonus.
I tend to buy and sell my cars every few months just for something new (if the wife permits me

), and I am up on my initial investment. e.g. I bought my last car and kept it for 10 months (which is unusual for me). Sold it at the start of January and made £200 clear profit. That's not bad for an investment 10 months on.
We're not talking big bucks here, but you can see how buying the right car can make a decent profit, even a long time after purchase.
I tend to find my cars on various classified websites, but not on autotrader. Never had a good car from Autotrader.