I need to set up a monetary system for my game however I am unsure how to craft an internal economy for that money. I want to make an rpg with a economy system that keeps the player for a time from just selling numerous of the same item to grind for money. So when the player sells to much of a certain item the buy price for that item will go down temporarily. Also, I would like to add a system where certain items go up in terms of what they can be sold for temporarily as well.
For example, in "Story of Seasons" I can sell items to vendors in order to get money, however if I sell to much of that type of item the price I get for it goes down. Also, there is a system where for a certain vendor or more the price I can get for the items actually increases.