“Do not let the buyer rejoice, or the seller mourn.” This is a reference to the standard back-and-forth haggling play between the buyer and the seller in the biblical culture, and when the sale is over, the buyer rejoices he got the deal at such as good price and the seller mourns that he will be ruined for selling so low. The point Ezekiel is making is that in the day of disaster buying and selling will not be as normal and even perhaps there will be no buying and selling.