Sell forty words for ten million coins
Each character wants to buy four letters to make four five letter words. The story indicates how to identify the words they require. Note that some characters will attempt to buy letters wanted by other characters, so you need to work out the correct order for sales.
Most characters will buy all four letters they want in a block (i.e. sold one after another) but this is not true for all of them. The last few letters need to be sold individually to maximise you return.