Feeling: I really don't like buying stuff from people who don't know what they are selling.
Theory: Things become crappier, unrepairable and more evil when the people who sell them no longer understand them.
Example: Computers. In the good old days (and before I ever used a computer), staff in computer shops actually knew how they work. Nowadays, try asking an employee of any store what the Intel ME is and does.
Other examples: Cars, Bicycles (especially e-bikes), and of course nobody knows knows how a smartphone works.