The first step is to check the pen against the actual model range. If the design on offer does not exist at Montblanc at all, a StarWalker with a marbled finish for instance, the matter is settled before any discussion of detail. A nib that does not belong to the series, or that carries the generic stamp “Iridium”, equally rules out an original. The StarWalker is among the most frequently copied Montblanc ranges.
The sales patter is characteristic too: the seller was given the pen as a present, claims to know nothing about the subject and therefore nothing about where it came from. Bids for such a counterfeit are no bargain but the price of a worthless object.
A listing can be reported, but only the rights holder can have it ended: Montblanc alone can make the auction platform remove it. That is why by no means every fake disappears from the listings, and why checking for yourself before bidding remains the most important protection there is.