eBay is definitely not in competition with any big name parts companies. The products you find in these eBay stores are of inferior quality. They are cheaper because they are not up to to the standards of other parts. Just look at where most of them are coming from, it probably says China, right? I would bet you that if somebody offered you a free set of parts from Eibach or Tein, you would take that over some no-name company selling on eBay.
I'm trying to get the point across that those items sold in the eBay stores are usually built of worse quality and are inferior. Not that they shouldn't be used.Hell, you can turbo your car with a turbo from eBay, but you can be damn sure that turbo wouldn't be nearly as reliable as a Garret turbo.
Now i'm just rambling and saying the same things over and over lol.