They were money changing temples. To buy anything to sacrifice, a dove usually, they had to face outrageous currency conversion rates to the temple currency. What outraged Jesus was those fees which were worse than usury in his "father's house." The unfairness was about the same thing the protestants objected concerning money for indulgences. You can see it was not banking, but currency exchange for "salvation," and an event where they would profit tremendously while abusing those of sincere faith.