Prediction Market - Margin call

[bytemaster]

I understand the blockchain will ahve its own short and long and margin calls.  I also know that the market is full of different prices.  Same way mt gox, bitstamp, btce have different prices.  From what uniform price and how is it fed into the specific transaction for a margin call to be iniated.  Is there a seperate prediction market overlay that feeds into the collaterized bitshare contracts?  Hopefully you understand my fundamental question somewhere between what I am saying.

All bids and asks are kept on the block chain.  The highest unaccepted bid is the price used to trigger a margin call.   This is the price at which the entire market agrees the value is ABOVE otherwise someone would accept the bid.   

No need to pull in outside 'trusted feeds'. 

[Bitcoinfan]

I understand the blockchain will ahve its own short and long and margin calls.  I also know that the market is full of different prices.  Same way mt gox, bitstamp, btce have different prices.  From what uniform price and how is it fed into the specific transaction for a margin call to be iniated.  Is there a seperate prediction market overlay that feeds into the collaterized bitshare contracts?  Hopefully you understand my fundamental question somewhere between what I am saying.