I have seen so many horror stories about knock sensors bought on ebay, I wouldn't touch 'em with a 10 foot pole.
Aside from the counterfeit "genuine" Nissan parts, there are plenty of sensors out there that aren't up to Nissan's OEM standards, for one reason or another.
Even if the part is indeed genuine Nissan, it doesn't mean that it works, or meets specs. Occasionally OEM parts get tossed due to mishandling (i.e., being accidentally dropped or damaged in shipment), improper storage (i.e., got exposed to salt spray during shipment), etc.
Knock sensors are too fragile and labor-intensive to be purchased on the cheap. Buy once, cry once.