How to buy on margin with bitcoin

[SheHadMANHands]

Noob question...

Are there particular sites that allow you to buy bitcoin on margin?

If so, can I hold a "margin position" if I transfer bitcoin to that particular site, or would I need to first sell that bitcoin and then rebuy it w/ margin (if that makes sense)?

Basically, I'm curious about increasing a current position, using current holdings essentially as collateral.

[orymh]

CampBX (https://campbx.com) plans to offer margin trading at some point. They haven't opened the feature on their live site yet, though. I haven't found an exchange that does (not since Bitcoinica, anyway, and they were a disaster).

Edit: I forgot that BitFinex (https://www.bitfinex.com/exchange) has margin trading, using funds borrowed from BitFinex depositors.