That's what collateral is for, to make sure you give the money back.  Do you mean to say that you will only show the lender that you have 1 million bitcoin scheisse, but not send them to the lender (or an escrow) to hold?  How is that supposed to make anyone "be sure" of anything?  Somehow the fact that you have a large number of worthless shitcoins is supposed make me feel confident enough to send you $1800?  A large number of worthless coins is still worth nothing, and it certainly doesn't mean you'll return the money.