Jesse Livermore has some good advice for Intrade, and Intraders

here:

The trading fee is 5 cents per $10 lot for trades between 5 and 95, and 3 cents per lot outside that range. The expiry fee is 10 cents per lot. Obviously Intrade needs to take a cut somewhere, but these per-lot charges distort the markets towards the extremes. For example, there is a bid out right now on Joe Lieberman to win at 0.3%. This probably overestimates his chances, but no one will ever match that offer. Why? If you bet against Lieberman at those odds, for every lot, you'd win 3 cents. Minus 3 cents for trading and 10 cents for expiry, for a net loss of 10 cents. This means there is no reason to bet against anything at odds less than 1.4%, because you can't possibly win money. This fee system also reduces the volume of trade substantially. Right now someone is offering to buy Mark Sanford at 1.5. If I took that bet, I'd be risking $9.88 to win $0.02, or laying about 500-1 odds. Meanwhile, the person on the other side of the bet would only receive a 65-1 payout if they won. Another way of putting that is that Intrade is charging an 87% commission on that trade. That's far too high, and kills most of the action in that range. A much less distorting system would be to charge traders a small percentage of each winning trade.

Fundamentally, it makes no sense to bet against anyone at odds of 10% or less. ... [Otherwise,] it would require a greater fool to come along and risk [even more on a long dated trade]. That's not impossible, but it's never a good idea to make bets which require others' stupidity in order to succeed.