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Yeah and it probably also has the best liquidity. If I recall correctly, they have some guide on their site that explains it all. Basically, with shorting you borrow BTC from someone on the site for a certain interest rate. Subsequently, you sell them and if the price declines you buy them back to realize your profit.
Do you have a referral number?
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I also think BTC is due for a correction but you can never be 100% sure. Shorting is dangerous. If your only goal is to buy more XMR why not just buy more XMR now instead of shorting BTC?
I'm a gambler, If I short and win and XMR prices stay sideways (I think that likely during my shorting period) then I get MOAR XMR!!!!
Mohahahahaha.

Actually I would get alot more if I didn't get sucked out on in a couple 1500 dollar pots last week and just bought instead. @$!$##!@