It takes a little while into the game before you actually need to start making money. There will be a scene with Horace that triggers when you have 5 of one ingredient. After that scene happens, Horace will be able to stop by outside in the evening on some days to trade with you, buying potions and ingredients from you, and selling back some that she may have.
As the game continues, more items will be able to be bought/sold and another location selling/buying higher cost items will open up.
But to initially start, you'll have to trigger that scene with Horace.