A great idea, but I have a few questions.

 1) When depositing Bitcoins, you say I have to sell them first in order to use margin, but with 1:5 margin I should still have
4/5 of the buying power to buy more Bitcoins, on a margin.

 2) If I sell short, and the price goes up, do I get a margin call?
     How much time do I have to comply?
     When will you automatically close the position?
     Can the losses in the account be bigger then the initial deposit?

 3) In case you suffer trading losses (e.g. from untimely hedging), what capital cushion do you have to protect my money?