All services technically buy parts at reduced prices. Usually cheaper than an individual can source the same parts from the same supplier. Normal policy in the business world.

There was a time where many things where kept in an active in house inventory a long time ago. So much variety became the norm it became impossible. In the day that inventory sat on the shelves. There had to be a profit on selling it. To justify the cost of stocking it.

Automotive service today is not what it was in the past. Try to find an ethical repair shop if you use them. This is where you can save money big time.

Rip offs have become the rule rather than the exception. Skill levels are all over the place as well. It also has reached the point where you should limit their use of after market parts in some areas of the cars. Too many of them are built and priced to just sell. With very limited service life expected. Absolute junk in many cases.