How to be sure you will pay me back with full interests? With banks, the solution is easy, the bank seizes the house and you can't do something. But the average Joe hasn't the same 'power' than a bank to collect a debt.
People can do same like what banks do. They can create a contract of agreement between the lender and the borrower just to make sure that everything will be okay until the end. A normal lender has still power even if he is not a bank. There are many things to think of.