You don't need to learn the required skill-set yourself. Because you pay someone to do it. If you do it yourself, it's another burden and it will be more inefficient at the start. Because professional sellers are better than that people of your own stuff but only at certain points.
- You can't train them the way you like
- You will lose a cut of your profit
- Vendors care less about your product than your own employees so you lose some expertise.