If he sells this to people whose hourly rate is $50, and hypothetically it would take each an hour to write, then he just created an economic surplus of $45 for

If he sells this to people whose hourly rate is $50, and hypothetically it would take each an hour to write, then he just created an economic surplus of $45 for each of those people, while he only took $5 for each sale. If he sells enough the income could allow him to create even more useful things for other people. There is nothing wrong with selling something you created. I'm an open source enthusiast. I run Linux on my MacBook Pro. Still, people should be able to sell what they created if they want to.