The people selling that garbage are really hurting you, the buyer in more ways than you know.
First off, the biggest problem is that they are, as you said, knock offs. Cheaper than original parts. These damn companies don't have to do any R&D etc, because the big name companies have already done it. These ebay sellers are selling crap they made, copied from another manufacturers brand name item. So in stealing someone elses design they can save because they are just copying it.
Example SS autochrome, I had always heard they buy factory rejects and cheaply chrome them up to make them look nice. THEY ARE TRASH! I saw one yesterday that was on for 6 months, its welds are cracked and leaking and the chrome finish is chipped off showing an unpainted header underneath.
Example Bomz and other no name CAI setups. They simply buy an AEM CAI and copy it then sell it cheaply. But then they break, because in not doing R&D they miss flaws the other manufacturer finds.
Second, When folks buy all this crap and put it on their car, they aren't buying parts from companies with respected names. This hurts because if the manuacturer doesn't sell parts it will not make new parts. For example, shitty short shifters. Maybe ford racing sees their shifters not selling. So now they decide to stop making them entirely. Oooops now the shitty copiers have nothing to copy! We the consumer now cannot buy a part because its no longer made.
If you are so poor you cannot afford a part then save your money.
I do not respect folks who would rather buy cheap no name shitty parts than folks who actually buy parts from companies we want to do buisness with.