I should have been more specific. Most stocks on the OTCBB and NASDAQ trade on far less volume than NYSE listed shares, so price volatility is an issue. Also, there are a lot more low price stocks on OTCBB and NASDAQ so small price moves can create large percentage changes.
Theoretically the person choosing a low price NASDAQ stock could influence its movement simply by purchasing a few shares near the end of the contest, especially if he or she already owns some of the shares.
It would be nearly impossible for one person to buy enough shares to influence the price movement of a NYSE stock - Bill Gates and Warren Buffet would be exceptions, but they probably aren't going to enter the contest anyway.
On another note, would you allow short-selling? After all, a gain is still a gain no matter how you get there.