During the time of the New Testament, Jerusalem was under Roman occupation (see Ancient Empires - Rome), and Roman money was in common use. The Jewish authorities however required that only Hebrew money was acceptable payment for the Temple tax. So, money changers were allowed to set up tables and benches (the English word bank is derived from the word bench) for themselves in the court of the Gentiles where they exchanged not just local Roman money, but also foreign currency from distant travelers, for shekels. Along with them were peddlers who sold animals, birds and various items for worship and sacrifice.
The money changers profited greatly from the exchange rates that they charged worshipers for shekels to pay the priests, and then from the priests to convert it back into Roman money - they were "cashing in" from both ends. They also profited exorbitantly from loans that they made - with interest rates up to 300 per cent. While there is nothing wrong with capitalism, or providing a useful and convenient banking service, these particular "bankers" were greedy profiteers who cared nothing about using God's Temple, and His worshipers, as a means to get rich.
It was the sight of this Temple "market" that triggered the only recorded incident in the human lifetime of Jesus Christ in which He reacted in a violent manner toward anyone -
"And they came to Jerusalem. And He entered the Temple [see Temples] and began to drive out those who sold and those who bought in the Temple, and He overturned the tables of the money-changers and the seats of those who sold pigeons; and He would not allow any one to carry anything through the Temple. And He taught, and said to them, "Is it not written, 'My house shall be called a house of prayer for all the nations'? But you have made it a den of robbers." (Mark 11:15-17 RSV)
Christianity Is Not A Business
"Do not lay up for yourselves treasures on earth, where moth and rust consume and where thieves break in and steal, but lay up for yourselves treasures in heaven, where neither moth nor rust consumes and where thieves do not break in and steal." (Matthew 6:19-20 RSV)
"There is great gain in godliness with contentment; for we brought nothing into the world, and we cannot take anything out of the world; but if we have food and clothing, with these we shall be content. But those who desire to be rich fall into temptation, into a snare, into many senseless and hurtful desires that plunge men into ruin and destruction. For the love of money is the root of all evils; it is through this craving that some have wandered away from the faith and pierced their hearts with many pangs." (1 Timothy 6:7-10 RSV)
"But thanks be to God, who in Christ always leads us in triumph, and through us spreads the fragrance of the knowledge of Him everywhere. For we are the aroma of Christ to God among those who are being saved and among those who are perishing, to one a fragrance from death to death, to the other a fragrance from life to life. Who is sufficient for these things? For we are not, like so many, peddlers of God's word; but as men of sincerity, as commissioned by God, in the sight of God we speak in Christ." (2 Corinthians 2:14-17 RSV)
"For whoever would save his life will lose it; and whoever loses his life for My sake and the Gospel's will save it. For what does it profit a man, to gain the whole world and forfeit his life? For what can a man give in return for his life?" (Mark 8:35-37 RSV)
Fact Finder: Can Christians serve two "masters"? Matthew 6:24