Hey, I trust your judgment if you say you know gens in and out. But what you describe (the weight, the feeling when screwing in/out etc.), do you think that this would hypothetically be sufficient in court as evidence?
It's not a rhetorical question, but actually a serious question?
Also, how do you know that the certificates are fake? Have you verified them with the manufacturer and they came back as fake? Or is it again just a hunch you are having?
I'm not trying to take your case apart, I'm trying to walk in eBay's shoes and attempt to emulate their perspective and reaction (I think that is in fact their reaction, since they obviously are not taking any action against the guy).
You'll need more than "feelings" and hunches to prove to eBay that the pens are counterfeited. For example, comparison photos by putting them next to gens which clearly show the difference. I think eBay would want to see something like that.