The term “ransom” is a business term derived from the marketplace. In this instance the item bought is people (so think of a slave market) and the price paid is blood. The people the Lamb bought were the property of another and now became the Lamb’s property.
9 And they sang a new song, saying, “Worthy are you to take the scroll and to open its seals, for you were slain, and by your blood you ransomed people for God from every tribe and language and people and nation,