If I borrow $50,000 against 45btc. When I payback $50,000 I get my 45 btc back. Regardless of value ($100 or $10,000/btc)
Yes, I did understand this. But for lending you need collateral - and at the current rate 45 BTC is enough for $50,000 - but when you default your loan, and bitcoin prices drops, what then?
If you're good for your money, it isn't a problem, but how can the lender be sure of it? (I don't have the spare money, so I can't lend you this)
Here is the info from Poloniex:
https://poloniex.com/support/aboutMarginTrading/ on this page there is the section
How Do I Offer Loans and Earn Interest? where it is explained.
You're right that this info is a little bit hidden (not easy to find), but lending is only for margin trading.