There is another dimension to this topic:
An important reason why parts purchased through mechanics are so much more expensive has to do with their supply chain.
It's not just the markup they impose. Typical markups would not even come close to explaining the gross price descrepancy. Some shops have zero markup on parts, but the prices are still high.
Mechanics have different priorities when choosing a supplier.
They value reliability, inventory, turnaround, and delivery service over price. They are not paying for the parts, and any supply problem that holds up their progress costs them big time. Heck, customers gripe whenever there is a parts holdup. Who wants to deal with that?
Just call a supplier that caters primarily to repair shops, and you will see what I mean. When all of the big retailers do not stock it, these outfits will be able to get it within hours (for a price). These folks are charging more because they have higher inventory costs, and because they are building rapid delivery services into their price.
You cannot blame the mechanics for this. It's just a market-driven reality.
1986 300E 5-Speed 240k mi.