For both 1st and 2nd trade runs, buying and selling stuff in this order should fill you up to the max, and over, pretty quickly:
You start to buy in Freedom Harbour and continue to buy as listed, selling whatever you bought in the previous place.
First Run:
Freedom Harbour: Wheat Bread
Valley of Whispers: Cloak
Ironclaw Woods: Agaric Mushroom
Heights: Bear Liver
Windcry Farm: Wheat straw
First Run (full profit 12000/10000 but takes longer):
Freedom Harbour: Wheat Bread
Valley of Whispers: Octopus Drinks
Heights: Nothing
Ironclaw Woods: Agaric Mushroom
Heights: Bear Liver
Windcry Farm: (Make SURE you do not go over 10000, be careful and sell slowly) Wheat straw
Second Run:
Freedom Harbour: Wheat Bread
Valley of Whispers: Cloak, Octopus Drink
Ironclaw Woods: Agaric Mushroom (Don't sell Octopus Drink yet)
Heights: Sell all (Should have 26k, if not, buy Bear Liver and continue on)
Windcry Farm: Wheat straw
Repeat the run.
Then it's back to Freedom Harbour, sell off your Wheat Straw, and you're done!
If, at any point, you get a message saying something about 'maximum currency', and you have over the number of mercury coins you were supposed to make (e.g. 23444/20000), then just head back to Freedom Harbour and hand in the quest.
I'm not sure if the money changes by server, so for now, take it that this is just for Storm.