First of all, it doesn't matter to me whether or not you're a TPF member or not. When people post links for authentication, I authenticate from the photos alone and am not swayed by my decision based on whether or not the person is a member of TPF.
The reason why I believe it is fake is because this style of Turnlock Zip Clutches only came out starting Spring 2008. The hardware that is on this particular clutch is from Spring 2007. Moreover, the only colors that this zip clutch came out in are Grass, Midnight, Earth (in shiny gold hardware) and the new Cranberry and Black (in silver hardware). The zip clutch in the auction in question does not fall into either category.
While I agree that "careful participation" is key, unlike you, I do not sell any Marc Jacobs or Marc by Marc Jacobs products. I only give my opinion to items where I am sure whether or not it is real or fake. Other times, I suggest potential buyers to request additional photos or information because I simply cannot give a definite answer based on the photos that are provided. I would never post anything that I am not 100% sure about.
I would be interested to know where you obtained this clutch from and if you still feel that I am accusing you of selling a fake zip clutch, I would love to see additional photos that prove otherwise.