The more sure you are about something the more you are willing to bet.
One of the reasons markets are more accurate than pundits is that some of them have inside information of sorts.
The reason intrade is useful for predictions is that the people who bet there do very careful research. For example when betting on American Idol vote-offs they look carefully at the data at dialidol.com. The people who don't look carefully quickly learn not to bet on intrade as they lose too much money. They also used to be able to look at download quantites on itunes for the previous week's singers songs. There is data out there that is tricky to find and the people who find it sometimes jump on intrade and make a bet. Instead of trusting someone to integrate all the data, it's easier to let the people betting at intrade do it for you. Also some of those people are friends and they chat about the data and how they interpreted it and argue about what it all means. The smarter people make the most money and keep at it, and they are the ones with their bigger bets that set the market price accurately.
There were hundreds of poll results but making heads or tails of it and including the electoral college of each state and figuring out which polls are biased and so on is a lot of work. Easier to just check intrade - let the experts (not pundits) make the bets.
If democrats tried to fool intrade by betting on Obama, then why did Obama's price go down so much after the first debate?
Maybe manipulating intrade isn't that easy in a big market like presidential win?