Why are some auto repair shops able to get away with being so dishonest and shady?

First, they charge ridiculous amounts for parts and manual labor.

How do they get away with charging you for repairs they never did in the first place?

How do you know they’re using a new part instead of an old, used one? Then they have the nerve to charge you for a new part.

Last, shouldn’t you be able to get a refund if the problem was not fixed when you got your car back?