This account is early in John whereas it is later in the Synoptics. In John, Jesus begins his ministry in Jerusalem, in the Temple. There are many Passover visits to the Temple. This is the first.
In this scene Jesus begins to confront the authorities. This theme will be seen throughout the Gospel. Also Jesus compares the Temple to the sanctuary of his own body. This is remembered after the resurrection.
At Passover time every Jew who could get to the Temple would get there. Some would walk great distances over many days. They would bring their offering for God – a lamb or a turtle dove. The offering prescribed demanded that the lamb be new born and perfect in every way. You can imagine that after many days on the road the lamb would not be the perfect animal it was when it left home. So there are dealers ready just outside the temple to trade you a new and perfect lamb for your wounded animal. These are the people attacked by Jesus for their extortion.
Jesus is asked for a sign of his authority. The prophecy of his death and resurrection is remembered after his death.
A NOTE ON SIGNS. The first twelve chapters of John’s Gospel are called the Book of Signs. In the language of John there are not miracles in Jesus life but signs. Events always have a deeper meaning than on the surface. As an example, what for Mark would be a few verses noting the healing of a blind man, would be for John a whole chapter (ch9). It is a sign that Jesus is the Light of the wold and that the Leaders live in darkness. In this context the word “sign” is used today.
“During his stay in Jerusalem for the Passover many believed in his name when they saw the SIGNS that he gave”.