A commercial seller with several thousand positive ratings who sells against an invoice showing VAT separately is comparatively safe: anyone systematically dealing in fakes will sooner or later be found out at that volume of business. A residual risk remains nonetheless. What tells you something are the usual yardsticks of the specialist trade, applied to buying online: are questions answered knowledgeably and promptly, are the service booklet and papers included, is there a right of return, and is the seller content for the pen to be examined by an authorised dealer? The reaction to that last question alone is revealing.
The most important rule is to buy only what you understand. On some models, the StarWalker in precious resin and certain Bohème versions among them, good fakes cannot be told from the original in the mostly mediocre auction photographs; such pieces are better bought from the specialist trade. It should also be reckoned with that condition descriptions often turn out too flattering, and that when buying second-hand one can virtually never choose the nib width, since by far the greater part of pens are offered with an M. Anyone who prefers EF or F will either pay a premium for it or have to have the nib changed later.
Anyone who has received a fake should approach the seller first and point out that selling counterfeits is not permitted in Germany; more often than not the pen is then taken back. If payment was made through a payment service, there is the further possibility of invoking buyer protection.