If I had to buy a part, I would try to make sure it really was bad. Where I worked, we had thousnds of spare parts as the plant needed to run 24 hours a day, nonstop. Usualy had one or more replacements for most everything. Those were small realys and I would usually put one in my pocket before I left the shop as it was in another building. I also carried some test equipment just in the rare case it was not that part. There was another part that was known to fail so took one of those with me, but as it took about 20 minuits to change it out, I would run a few tests to determin if it was the problem.
I did have a Toyota that started running rough. As I am not a very good car mechanic, I threw a few logical inexpensive parts at it. Plugs, wires and coil. The hint page on Autozone pointed to a mass air flow sensor, but it was around $ 500. Took it to the Toyota dealer thinking they could test it. Two weeks or more later they finally decided it was the sensor and put in a new one. Not sure what kind of mechanic they had, but did not seem to be much of one.
Then there are the ones that do not have a clue and do not seem to know what to do, so they just change parts blindly.