I got some 10W LEDs last week. I bought two small batches to see if they were different and did the "Big Clive" test.
One batch was perfect. One batch wasn't. They both cost about the same on eBay. Go figure.
PS: The good ones came in individual bags. The 'bad' ones were just rolled up in a piece of foam.
The good ones come from the factory, the bad ones come from the factory's dumpster. When they both end up at the Shenzhen markets, the cruddy one is the same price as whoever got it doesn't know what it's worth.
Its much more nuanced than that.
There are no factory dumpsters in China
Everything is sold, rejects are labelled as second grade and bought by same people buying normal product. They end up next to each other on the shelf, final price difference (if any) depends on your relationship with particular seller. No relationship means you get the fake/rejected ones, good relationship gets you rejects at a steep discount. Being an ebay reseller you dont give a toss about quality, only price matters. $.02 too high and listing gets pushed to second page = nothing moves. This means you buy real product only when rejects run out.