I have done quite a bit of research on this a while back. From what I found, most of the Sensors and lenses come from the same place (you know) regardless if the hood/helmet states it is made is USA or other Country... Look as the specs for the helmet response time, etc as that can distinguish between cheap crap and inexpensive. Also, I would recommend you get a dual adjustable (however, I am pretty sure that most are now a day). I personally would really avoid Ebay as you should be able to find a good quality helmet locally and will know what you get. ...that's just me though.
I had a Cheap 1st gen Auto Dark helmet (single Adj only) and the second time it did not auto darken (after a couple of years), I did the research and bought a new one. I (in the US) was looking at the Miller, Snap-on, Lincoln, etc. I found a Helmet from Lowes, of all places, (which is a sort of building and supply company that has their own "branded" tools for those not in the US). The lens and sensor is comparable if not the exact same as all of the very expensive ones I researched. In fact, side by side it is impossible to tell the difference with one or more of the afore mentioned brands. In research SpeedGlas and Jackson Helmets (USA) seemed to be revered as the best of the best, but being and engineer, I would need to see proof as to why they warrant 3x the price and I could not find it in the tech specs. I would love to see data on why I am wrong.
Anyway, After using my new one for about 2 years, I can say it is awesome. The Response time is instant and it has never let me down (TIG, Mig etc). Even in different lighting conditions..
Now, with all that said, a dirty outer lens (from splatter, etc) can/will cause sensor issues (regardless of how expensive of an auto darkening helmet).
Regards,
Shawn