What are the illicit earnings according to the four (Sunni) schools of thought, and what are the reasons that necessitate the prohibition of things or earnings?

An important principle in transactions is that the default is permissibility unless they involve something prohibited. Prohibited earnings are many, including: usury (riba), consuming people's wealth unjustly, bribery, selling what one does not possess, 'inah sale (buy-back agreement), and selling a fetus in its mother's womb. The prohibition of these matters can be attributed to three things: usury of all kinds, consuming wealth unjustly such as through cheating and deception, and ignorance and excessive uncertainty (gharar).