You cant do it trust-less.
If you want to buy a car for $10K and you have $5K, you will need to borrow the other $5K. If you post your original $5K as collateral, you gained nothing. You still only have $5K because the other $5K is tied up now. There is no such thing as a trust-less loan. Banks will loan you money because they have recourse through the laws of the land. For example, they can take your home, car, garnish your wages or put liens on property if you do not repay the loan.