I don't even want to go into the idea that a big seller of gear sells 20ph of duds and keeps 20ph of good gear intentionally.
As I could see it as a way to hurt pools and help yourself at solo mining.
While the bigger sellers may not do this thinking they would be found out.
The fact is right now pools have zero protection against it.
As difficulty rises, it makes it harder for the small guy to find out if his hardware is dud, especially if they participate in pools.
Given the douchery of almost EVERY mining hardware manufacturer out there, I would not be surprised if garbage is being sold. The only people who can truly test the HW are the manufacturers. When you manufacture silicon and assemble the chips, you have test programs that allow you to test the circuit.
You would have to be an idiot manufacturer to not test the goods. You know which chips are stable / good, and which are bad.
ONCE you seal up the device for sale, you put a "warranty void" sticker on it, but that prevents many hobbyists from opening it up and testing themselves (if they know the pin sequence and codes), so they operate on faith that it is good hardware.
In a completely random universe, the pools should be receiving coin proportional to their hashrate. I have never bothered to check the math... but it shouldn't be hard to do.