Tradesports are an exchange (like betfair) - you can put orders in to buy/sell at any price. Each contract is worth 10 dollars if it expires "true" and they are priced on a scale of 0 to 100. The fair price is therefore just the probability in percent of it coming true. This pricing makes it easier in my opinion to manage your position during a trade and to do things such as sell five contracts that are 10 percent likely to come true to finance buying a 50:50 one.
I'm fairly new to this stuff and just trading in low volumes but having looked at a few different sites and after reading the posts on here, tradesports is currently my favourite. I notice that nutter punter has a few posts saying that tradepsorts/intrade are crap but without any reason - I'd really like to know why he thinks this. Personally I think that an exchange with independent market makers (it actually looks like two rival market makers on their Dow contracts) who quote with volumes of hundreds of ten-dollar contracts and who don't have any unfair advantage over you is about as good as it's going to get. If you look at their stats there are twenty or thirty thousand or so Dow contracts being traded each day with numerous trades for 100 or 200 lots going through - so some "big players" clearly think it's a decent site.
