Lumens specs when buying a flashlight/torch. Why maglite has only 680lm but cheap ones have 900,000?
Well in your eBay-Link they are providing enough material to debunk themselves. They say they use a Cree XM L2 LED. So let's just look up what that thing can output.
Datasheet XM L2 LED and we see: even if it is driven with 2000 mA - which is quite the stress on the battery - it outputs 600 lm.
And they kindly provided a picture showing that they only use one LED and not multiple.
So they are lying or they measures the first production batch in unison and forgot to say they used 400 of these things.
Generally, you can expect around 100 lm / W and a handheld device is probably using 10 W maximum (okay might be 20 W or so with a good battery), so anything beyond 2000 lm is just unrealistic.
If they tell you the 4000 mAh battery will last 6 hours, you can calculate the wattage:
4000 mAh / 1000mAh/Ah * 3.7 V / 6 h = 2.5 W.
So 250 lm would be a realistic guess for the brightness. And yes, the 4000 mAh are faked as well - currently 18650 LiIon are around 3200 mAh maximum.
Okay, so how does the "brightest torch" from wicked lasers hold up with these estimates?
They claim 4100 lm using a 100 W OSRAM halogen bulb. The datasheet of that bulb tells us it emits 2800 lm. (Edit: the reflector does not affect the luminous flux, sorry about my mistake I often mix up the luminous units) I'm not really sure how they arrive at the 4100 lm figure - with 100 W it would be 41 lm / W, which is unrealistic high for a halogen bulb. So probably a bit of overadvertisement to stay with the claims of the cheap competition.
What about the power of this thing? They claim 100 W which sounds ridiculous. But they use 4 18650 batteries in series and give a low estimate of 20 minutes lifetime. Sadly the capacity is not given, but this is a really high current application, so the capacity will be a bit smaller - I guess 2500 mAh for a high quality cell.
4 * 2.5 Ah * 3,7 V / 20min = 111 W
So the math actually checks out and I think that it does what they say.
TL/DR: It's a scam.
Current LEDs that work well in flashlights have luminous efficacies in the 100-200 lumen/Watt (roughly, not factoring in driver losses etc).
Thus a 900 000 lm flashlight would require 4500-9000W of power... thus a huge battery and about the same cooling fan as a hairdryer.
Also LEDs for flashlights are usually rated between 1 and 10W and a common LED power for a flashlight is 3W. So if you look at the flashlight and only see ONE LED in it, and it is rated for more than 1000-1500 lm, expect trouble!
Then you can check for battery lifetime. An AA 1.2V 2500mAh alkaline can deliver about 3 Wh (Watt Hours) of power which means 3W for 1h or 1W for 3h, you get the idea. So a flashlight with 300 lumen (about 2-3W) with a battery life of 2 hours on 2 AA batteries sounds good. A bit optimistic considering AAs lose capacity at high current but... not in scam territory. If it is advertised for 1000 lumen and 10 hours battery life on 2 AAs, then... you know something's off!
Lumens describe the total amount of light out of the flashlight. Candlepower (candela) is a misleading number as it describes lumen/steradian, ie light flux in the beam. The same LED, same lumen, will have much less candela with a flood optic than with a tight beam optic, because the tight optic concentrates the light more. Using candelas is a good way to get impressive numbers which look good!
Also cheap flashlights tend to have gotchas, like no spring contacts for the battery, so when you shake it a bit it will turn off or switch to blinking mode.
In other words, go to a flashlight geek review site and pick one by a good manufacturer.
So why on auction site are they allowed to state 900,000 lumens.
How is ebay supposed to check?
If you want a good light, first decide on the batteries you want. 18650 Lithium are better, especially in the cold, but require a specific charger. NiMh AAs are more convenient, but lower capacity.
Note that in some regions, a comma is a decimal separator - the manufacturer could claim that the nominal light output is 900 point 000 lumens. Obviously, there is questionable faith in gaming such conventions.