Apocalypse 10:8-11; Psalm 118(119):14,24,72,103,111,131; Luke 19:45-48
Jewish people had an obligation to go to the Temple every year and make sacrifice to God. The sacrifices were animals, and the animal reflected the wealth of the family on their pilgrimage. For all sorts of practical reasons the obtaining of an animal to offer was taking place in the temple (in the outer courtyards) where of course a flourishing trade developed. It is likely that this was a somewhat corrupt market, as there was an insatiable demand for animals, and supply was no doubt managed carefully.
But Jesus was going to make of himself a single, lasting sacrifice that would endure for all time, and the driving out from the temple was part of that message. In Mark this was a violent affair, and could be mistaken for being the reason that the authorities wanted to be rid of Jesus. In Luke it is made clear that Jesus prayed every day in the temple, up until his own sacrifice. It was Jesus' preaching that the authorities could not stomach. But that same preaching was lapped up by the people who listened to it for it contained the message of our salvation.