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This is one of the innovations that BTC exchanges have created. No fiat broker allows you to lend out funds to other users for the purpose of margin trading.
The guys at Bitfinex and Poloniex (I´m not exactly sure, which of both exchanges had P2P margin funding before the other), who came up with this idea
are really bright. This has brought in a huge amount of users, which would otherwise have used alternative exchanges.
Let´s be realistic here. If a user is already lending out at one of the exchanges it is much more likely that he will conduct his trading at the same
exchange as well due to pure convenience.