I can give you an example. Everyone knows that if they get a C8 from Convoy (host or complete), it’ll be the Real Deal™. But if you get a C8 clone off Fleabay or Amazon, it’s a crap-shoot.
A few years ago, I got some C8s and ’502s which were pretty fantastic. Bright (at least 700lm), an LED that looked like a genuine Cree XM-L or at least was a good clone. Build and fit’n’finish were top-notch (one of those ’502s I use as my EDC with a different drop-in). While the light was definitely cool-white, it wasn’t an ugly Angry Blue color.
About a year later, I got a whole bunch more C8s for use only as hosts, but wanted to see how they’d be anyway when complete. Utter garbage. Plastic reflector (not aluminum), no O-ring between lens and bezel, cheap crappy LatticeBright chip that was about the size of an old XP-E, definitely Angry Blue light, and barely 70-80lm out the front. Oh, and the pill was also cheap aluminum, about half the height and a third of the weight of an actual brass C8 pill (type 1 head).
It was depressing.
I noticed that in general, “quality” has been a race-to-the-bottom. One uses a cheaper LED to reduce costs and try to sell a few more lights, and the next bozo uses an even cheaper LED to cut costs more and sell his lights for a few pennies less.
Oh, but these 70lm lights are touted as 1000lm!, 1200lm! 1500lm! 3000lm!, ie, supposedly so many lumens coming out that one little LED chip that it’d have to be cryogenically cooled to liquid-helium temperatures to get that much light out of it.
Ie, blatant lies and outright fraud.
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I bought a couple of D26 drop-ins that were advertised as 3V-18V, even with a sticker slapped on the side of the pill stating so quite boldly. Well, I tried hitting it with ~8V from a pair of 18350s instead of a single 18650. It got very blue and very dim for the 1-2 seconds I had it connected. Its “current regulation” was no more than a single small-value resistor! So of course, hitting the poor chip with double the voltage must have almost fried it. To the chip’s credit, it still lit up well from a single 18650 after that.
So what went from about a 4-star rating on Amazon turned into a 1-star scathing review calling out and naming the seller, so that no one else gets, well, “burned’.
And that’s another problem, that on Amazon, the “same” item can be sold by a dozen different vendors, some with good merchandise, others with garbage. That’s why I try to name the vendors in my reviews, both good and bad. So if LuckyLady888 sells something good, I’ll say so. If HappyLuckySunMoonStar sells garbage, I’ll say so, too.
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I went out of my way to never ever get an unknown clone of any C8, ’501, ’502, etc., online anymore. It’s not worth the crap-shoot I mentioned. True, with Amazon you’ve got their a-to-z guarantee, but it’s still a chore to do a return. (Unless of course they just refund the bux and don’t expect a physical return, but how often does that happen?) From other sellers (FT, GB, etc.)? You need to snap pix, take videos, jump through the requisite number of hoops, before a year and a half later you eventually get a refund. 
So now, I’ll get Genuine Convoy™ hosts (S2+, S5, C8, etc.) if I want to roll my own. FT is pretty good for that kind of thing. Actually, really good. I never had a problem with any hosts I got from them. Or I’ll get name-brand lights like from Jetbeam, etc., if I want off-the-rack.
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Oh, something I just saw today… I wanted wire. Simple, plain, everyday wire. 18ga, individual or red/black pair, was open to anything. One seller, AudioPop or something, was selling “18ga” wire and made a very bold and big note, “Wire gauge is not AWG!” or somesuch. Well, for good reason. They got slammed by lots of people yelling fraud, that their “18ga” wire is actually extremely thin, more like 20ga if not 22ga! Wtf?!?
That’s like saying you’re selling a “12-inch ruler”, only “inch” is not “US-inch”, so the 12” ruler you get is only 5½” long!
Ah! Here ya go… Read it and weep: Amazon.com
But crooks like that get away with it! At least long enough and often enough to make a profit at it.