Active electronics is one are you need to be the most careful. Pop that cheap relay open and look at the components - ten to one, they'll be completely potted in opaque material that hides the fact it's using an underrated thyristor. It'll work... as long as you don't push too much current through it. It might also catch fire.

You can't even trust what's printed on the ICs. Seriously, look into chip counterfeiting, it's a real problem.

That's why active components from Digikey and Mouser are so much more expensive than what you see on Amazon or eBay. They can trace the components back to the fab where they were made. You pay more, but you know you're getting what you ordered.