I think the difference here is that you're taking this from the perspective of an actual engineer, and I'm a hobbyist (and I assumed the same of OP; I hope Real Engineers aren't getting parts off eBay). Which means that, yeah, I'm happy to handwave a lot. By software analogy, I write a lot of shell scripts and python, which is passable but hardly rigorous; if I wanted it to be Correct, I'd break out coq or write in Ada or something, but I just want something that usually works because the stakes are so low. Of course counterfeit parts aren't reliable, but if failure is an option then they're good enough.