In Mark's chronology (chap. 11), Jesus enters Jerusalem on Sunday, looks around, then returns to Bethany. On Monday, then, he returns to Jerusalem, curses a barren fig tree on the way, and then overturns the tables of the moneychangers in the temple. Then in the evening they return to Bethany again.
The chronology of Matthew, Luke, and John is a little different, which is why we cannot be absolutely certain about what happened on which day. In Matthew 21, Jesus seems to overturn the tables on the same day that he entered Jerusalem, in contrast to Mark. In Mark also, Jesus curses the fig tree right before overturning the tables on Monday, and it is only on the next day, Tuesday, that they find it withered. In Matthew, Jesus curses the tree and it withers immediately, the day after the temple action.
Luke seems to follow Matthew's chronology at this point, but does not have the story of the fig tree. John seems to move the temple action to the first year of Jesus' ministry. It is of course possible that Jesus did this twice, but after you have compared the gospels even a little in detail, it's hard not to come to the conclusion that they intentionally move things around from time to time. This shouldn't be a problem for us. It has to do with our (faulty) expectations rather than any problem on their part.
There are differing interpretations of Jesus' action in the temple. Was Jesus angry because they were selling things in the temple? But they had to sell things so that travelers would have something to sacrifice. Were they selling in the wrong place? I really don't think the relevant texts will bear this weight even though this is often said. Was it a symbolic action, planned out and meant to symbolize God's coming judgment of Israel and even the destruction of the temple itself? I think it is very likely that Christians read it this way, especially after the temple was destroyed.
The key to me at this point (meaning after this round of reflection ;-) is Jesus' quotation of Jeremiah 7:11: "Has this house, which is called by my name, become a den of robbers in your sight?" (NRSV). The context in Jeremiah is a prediction of the temple's destruction because Israel's leaders have oppressed "the alien, the orphan, and the widow" (7:6), which fits in with Jesus' focus on the poor and powerless in his earthly ministry.
So I find the spirit of John's presentation to fit very much with a general distaste Jesus must have had for the wealthy, profiteering atmosphere he must have seen before him. It couldn't have been disapproval of the selling per se, because that was necessary. But the spirit of what he saw must have represented to him everything that was wrong about the values of Jerusalem.
His action was thus an indictment of the spirit of Jerusalem's leadership and a foreshadowing of coming judgment. It is no surprise that Mark "sandwiches" the event inside the incident with the fig tree. Just as the fig tree did not bear fruit and withered under Jesus' curse, so the Jerusalem of that day was eventually judged for its failure to bear the kind of fruit pleasing to God.