After having purchased a fake Sennheiser e845s on ebay (a satisfactory ending as I returned the item to the seller and had my money reimbursed), I thought I'd share my experience and show you how to spot the difference (apart from the thin reproductive sound and the typographical errors in the brochure that accompanied it).
There is a noticeable difference to the shape of the mesh grille of a genuine Sennheisers e845 compared to the fake one I received.
The mesh grill on the genuine e845 is elongated with relatively flat sides and a blunt end. On the fake e845 the mesh grille had more flared sides that tapered to a more rounded end.
Genuine e845 Fake e845
I would suggest looking closely at the actual photographs of these microphone and compare them to the photos of the Sennheiser microphones on their corporate website before you decide to bid.
I trust this article will be of assitance.