Also,
CampBX can execute advanced trades and margin.
Not today. Their site has had that teaser since it launched more than a year and a half ago, but they currently do not have the margin trading, stop loss feature, etc. Because they do accept deposits of USD funds through Dwolla there is one technique that some use to gain leverage for trading bitcoins. Dwolla Instant is a method in which Dwolla provides a loan of up to $500:
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http://help.dwolla.com/customer/portal/articles/433827-instant-faqWhile hopefully this access to credit isn't being used to speculate by those who would not be able to afford the losses should the exchange rate move unfavorably for that speculator, it does become a tool allowing a trader to short in an alternate way.
There is demand for "dollar loans" in which bitcoins are the transactional currency but the loan agreement specifies payment of interest and the return of principal to be calculated in terms of dollars. So by taking a USD loan through Dwolla Instant, buying bitcoins with those borrowed funds and then lending them out to a borrower at a higher interest rate then that person is in effect shorting bitcoins using leverage. If the speculator is correct and the exchange rate drops by the time repayment of the loan occurs, there will be a net positive balance of bitcoins after settling (repayment) of the Dwolla USD loan. Of course, there is risk of default by the borrower so this method of shorting should only be considered by those with experience in lending.