I knew someone who won a similar stock trading contest held by a radio station about 30 years ago. He put all his pretend money into a low volume penny stock. Then in real life he bought enough of that same penny stock to raise the stock price substantially. He'd somehow calculated that the cash prize would exceed the cost of manipulating the stock. IIRC the stock price got another boost when they announced his winning trade on the radio, enabling him to make a tidy profit.