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I mean lending out Bitcoin or other assets to traders to use and earn interest on it. Like Poloniex, Bitfinex and QUOINE

Oh, that's a risky business. Done that on one of those Bit Lending P2P sites a long time ago, BTCJam, only got parts of my investments returned again.

Now I am in two minds whether to keep lending or not.

I was under the impression it was safe to lend because the trader borrowing the funds would automatically be liquidated by the system if there position fell below a certain %.

Obviously there is the potential threat of the exchange being hacked or going rouge but 30% APR return on Bitcoin is so tempting.

Don't take my word for how lending works, I don't know anything on how it works on exchanges, just that my own experience of it at BTCJam was really terrible :D