The trader in the elven forest always has infinite amounts of elfroot available. The trader in the mage's tower always has infinite amounts of lyrium dust.
Buying the lyrium dust and other other necessary ingredients for a potent lyrium potions (the other ones are available in infinite amounts from the dwarven trader in your camp) costs less than what you sell the potions for. So you can basically get all the money in the world, and then spend them on whatever you need - like as many potent potions as you want. This is kinda game breaking, though. If you're a dwarf, there's a dwarf plot character who's a trader in denerim and gives slightly better prices than anyone else