The ones on eBay *may* be genuine Nissan, but they certainly aren't genuine 350z knobs and the seller doesn't claim this either, he just says they fit.
TBH they look a bit crap, there must be better out there than that.
The seller actually says "When fitting to a 350Z, some of the gear shaft will be left exposed, as the gear gaiter does not attach to the gear knob. This may give a more racing look." Good try mate