They were money changers. In other words, they were converting money from one countries to another and cheating the people. These weren't just any people, they were people coming to the Lord's temple. Then the people selling animals, they used the Gentile area to hold the animals. The Temple had a special area for Gentiles to come, because all were welcome in God's temple. That area was taken from human use and used for livestock. Another thing is that God required a spotless animal sacrifice. The people selling animals would look at someone's sacrifice and say things like "Is that what you are going to sacrifice to the Lord? Look, just buy this goat from me, it's better, it will be a better sacrifice." Then they would charge an large sun plus the other guys animal and then sell the first guys animal to the next guy through the gates. The temple grounds weren't being used to make money, they were being used to make money through very underhanded tactics. That's why Jesus was mad. God required that Jews come to the Temple at least once a year and the people of Jerusalem took advantage of them.