What is the ruling on selling a service purchased from a third party at a lower price and keeping the difference, knowing that the service is intangible (such as website design)?

There is no objection to the questioner designing websites. What does not differ with the difference of persons, it is permissible for the employee to hire another to do it, whether for the same wage, or less, or more, unless the first employer stipulated that the employee do it himself.