Great way to get a counterfit knock-off part is Amazon. Rock Auto may not be an authorized distributor for what they sell. This means if you get a Bosch glow plug from Rock Auto, they are NOT an authorized Bosch dealer, and it breaks off in the engine requiring the engine to be rebuilt/replaced, well, NO WARRANTY. You eat the bad glow plug AND engine that would have been covered otherwise.
I accept the fact Rock Auto gets leftovers from other vendors. so far nothing I have purchased in the last 15 years has failed.
I also accept the fact that many people are mechanically insecure and they need to pay for things that they are comfortable with.