You could start a lending business with strict collateral rules as mentioned in the lending section.People follow the rules and the do pay you back so there is good chance for you to double your money.
yes but sir if people have collateral they usually just sell for the money they need 
No, that's the idea of colleteral. You don't want to miss the item, you need it still or want to keep it for some reason.
You put it as colleteral in case you cannot get enough money to pay back the loan. In that case, you lose your colleteral, which is something you don't actually want (also, because the colleteral should be worth more than the loan you are asking).