Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

If you buy something and sell it later on the same day, how do you calculate 'investment'?
Not sure if your question is on topic, but the investment is only $9 because that is maximum amount of money the merchant ever needed to start up the business. He put in $9, started turning a profit, and never looked back.