The part-worn problem: where corners actually get cut
The corner-cutting is not always so blatant. It also looks like a shop fitting your tyre without checking the inside of the casing after a puncture, plugging a sidewall injury that legally and technically requires the tyre to be scrapped, or reusing a rusted valve stem to save five minutes. String plugs pushed in from the outside without demounting the tyre are a roadside expedient, acceptable to reach a workshop, not a permanent repair. A proper repair means demounting, inspecting the inner liner, and fitting a combination patch-plug from the inside, with the tyre rebalanced afterwards.
Your defence is process, not suspicion of everyone. Insist on seeing the removed tyre and the damage before agreeing to a repair-versus-replace decision. Ask whether the repair is a patch-plug from the inside. And when a puncture sits in the shoulder or sidewall, accept the replacement verdict: no honest fitter will warranty a repair there, and any insurer investigating a later blowout will find it.