I am sure I have posted about this before, but this insight from N.T. Wright on this passage well worth repeating!
12 Jesus entered the temple courts and drove out all who were buying and selling there. He overturned the tables of the money changers and the benches of those selling doves. 13 “It is written,” he said to them, “‘My house will be called a house of prayer,’ but you are making it ‘a den of robbers.’” (Matt. 21:12-13)
It isn’t the buying, selling and money-changing he’s objecting to in itself. When he says, ‘You’ve made it a brigands’ lair’, the word brigand doesn’t mean a thief. ‘Brigands’ were revolutionaries, people who believed so strongly in God’s coming kingdom of justice and triumph for Israel they were prepared to take the law into their own hands. They were the violent ones Jesus had commented on earlier in the gospel (11:12). The Temple itself, instead of being regarded as the place where Israel could come to God in prayer, had come to stand for the violent longings of the ‘brigands’ for a great revolution in which the kingdom of God would come by force. It was everything Jesus had opposed throughout his lifetime, not least in the Sermon on the Mount. Now his warnings against ‘the house’ were to come true.
Wright, T. (2004). Matthew for Everyone, Part 2: Chapters 16-28 (p. 72). London: Society for Promoting Christian Knowledge.