Chapter 2: Topical Index
In all my typing, I failed to acknowledge the very real possibility that the physical location of the money-changers was an issue. While I am aware of no Mosaic injunctions against buying or selling in the temple, I am neither aware of any explicit Mosaic instrution or permission to sell within the temple compound.
But I'm not familiar with evidence that the poor were being cheated; the Mosaic commandment seems to have been for a proportional exchange--if the pilgrim lived too far away to reasonable transfer the proscribed offering, he was instructed to purchase whatever type of sacrifice he wanted with his money, and to offer that, in proportion to the worth of the commanded sacrifice. At least, that's what the Law seems to be saying:
Deuteronomy 14:24-27 (New King James Version)
24 But if the journey is too long for you, so that you are not able to carry the tithe, or if the place where the LORD your God chooses to put His name is too far from you, when the LORD your God has blessed you, 25 then you shall exchange it for money, take the money in your hand, and go to the place which the LORD your God chooses. 26 And you shall spend that money for whatever your heart desires: for oxen or sheep, for wine or similar drink, for whatever your heart desires; you shall eat there before the LORD your God, and you shall rejoice, you and your household. 27 You shall not forsake the Levite who is within your gates, for he has no part nor inheritance with you.
At the end, there is instruction for the pilgrim to remember the priest--if they "charged" a bit extra for selling sacrifices, it would seem to me to fall under this instruction to provide for the Levites. But that's only conjecture; the specifics of the sacrificial system have been lost or obscured by the passing of time and a great many different remembrances of an ideal system. I would be glad for more information, certainly.