Unlimited gold
First available after you get the alchemy pot.Step One: Buy as many medicinal herbs as you can (at the earliest point in the game to do this I was able to buy around 140 for 1,120 gold).
Step Two: Go to the alchemy pot and make as many (70+) Strong Medicine (If you don't have the recipe it's medicinal herb + medicinal herb).
Step Three: Sell the Strong Medicines for 88 Gold each (6,160 g) Rinse and repeat with the following recipes (you have to change it up or the vendor will wise up and not pay top dollar for a particular item). Strong Medicine + Strong Medicine = Special Medicine (4x medicinal herbs selling for 170g in the end) Medicinal herb + Antidote herb = Strong Antidote (95g) Medicinal herb + Antidote herb + Moonwart Bulb = 280g Med + Med + Moonwart = 148g Moon + Moon + Moon = 308g
There's a few more high-earning recipes, but using all of the above will yield millions of gold at a fraction of the time it would take to grind it killing monsters.