Schaff's Bible Dictionary
CHANGERS OF MONEY, or MONEY-CHANGERS. Matt 21:12; John 2:14. When Judaea became a province of Rome the Jews were required to pay taxes in Roman currency, while the annual tribute for the service of the sanctuary was the half-shekel of Jewish currency. To exchange the one for the other was the business of the money-changers, like the business of modern brokers.
They stationed themselves in the courts of the temple, the place of general resort for strangers from every part of the land, and their oppressive and fraudulent practices probably justified the allusion of our Saviour to a den of thieves.