Shopkeepers buy an item from a wholesaler and sell it to customers. How would you build a model to describe their profit?

Shopkeepers buy an item from a wholesaler and sell it to customers. How would you build a model to describe their profit?

Shopkeepers buy an item from a wholesaler and sell it to customers. How would you build a model to describe their profit?
Shopkeepers buy an item from a wholesaler and sell it to customers. How would you build a model to describe their profit?