Counterfeits, mostly. You will never find a legitimately sourced component on amazon.

Maybe that doesn't matter to some people, but it guarantees GIGO.

Most of the stuff I buy is just $RANDOMNAME_MADEUP_BYIMPORTER that was produced to hit the lowest cost point on amazon, and which will work fine in nearly every case I subject it to. For example, my BOJACK 50 values 1350 Pcs Resistor Kit. Do you want me to go measure a sampling of those and compare it to the equivalent kit (or pages of components) from DigiKey?

Not sure about the counterfit, the PSU I just got is from ARESGAME which appears to be a California company, and from every detail of the product, it appears to be legitimate. I just popped it in my main desktop computer (cyberpunk 2077 was crashing/windows rebooting when I hit 400W). I fully expect this device to work as expected for the next 10 years I will operate this machine. Same for the $1K Dell monitor I just bought. This thing was obviously made for dell and sold by dell with top grade components.

A small fraction of the stuff with $RANDOMNAME I've bought on amazon has been total and complete crap, but once should probably be suspicious any time something is sold for 1/10th its ostensible manufacturing cost.

For people like me who can't remember all 100k acronyms: "GIGO" is "Garbage In, Garbage Out".

I agree with the sentiment though - a lot of stuff on Amazon are not exactly what is written on the tin, and so when you read official spec of a component you may get stuff that's way off - noisy opamps, that sort of thing.

Professional here: I hate Amazon and die a little inside every time I hear about people buying electronic parts from them.

So I am not trying to push an agenda when I say that they're fine for generic parts. You are probably not going to cry if your 0.25W resistor smokes itself at 0.2W instead of 0.3W like the name-brand part. Your random 1N4007 is probably still a 1N4007; it's not like they're hard to make. I wouldn't use it on a 999V line, but I probably wouldn't do that with a genuine 1N4007 either.

Where Amazon starts to get dodgy is when you are dealing with delicate parts: precision amplifiers, precision resistors... really, precision anything... or anything where you will need more of that exact thing in the future. You'll get something when you order from Amazon or AliExpress. It might even be the right something. But good luck getting it again later!

OK. This is a reasonable answer; Amazon is good for 95% of the market and you shouldn't attempt to use it to fulfill the remaining 5%. That's correct: I use digikey or mouser to order precision and power resistors (for the 5% of the time I need either of those instead of a generic cheapie). I use mcmaster for some parts that amazon doesn't stock (or what they stock looks dodgy).

But over the years "what is precision" has changed. For example, I can buy micrometer stage on Amazon for $100 that is as accurate as the $10000 stage from 10 years ago.

Also, I think most electronics parts are engineered with 3X so I wouyld expect my 0.25W resistor from amazon to smoke itself at 0.75W and the one from digikey at 1W.