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Margins between buy/sell

So just wondering what causes larger margins between buy and sell? For example 

the topic "Which candidate will see the worst polling impact from the first-tier CNN debate?", the buy on Donald Trump is 49 while the sell is 41. Why the 8 percent margin? Why would what I bought only then be able to be sold at a much lower price? Thanks for the help, new to trading.

The "margin" you are referencing is called a spread.  The details can get rather complex, but in simplest terms, wider spreads indicate a more illiquid market.  Not enough players, not enough contacts etc.

Large spreads between buy and sell prices usually occurs when there are relatively few traders. 

In this case, it means that another trader perceives the probability of Trump having the worst polling impact as 48% or less (and so is willing to sell you a YES share at $0.49, i.e. more than its perceived "fair value"), while another trader thinks the probability is 42% or more (and so is willing to buy a YES share off you at $0.41%). 

The prices tend to converge when you have more people active in a market, and the margins become much smaller.

If you look at the "most predicted" market, you will see that the spreads between buy and sell prices are typically only one to two cents. (The exception is for shares with extreme probabilities, e.g. where the buy price for either yes or no is over $0.90. This is because 5% trading fees tend to screw things up a bit.)

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