The first step is to attempt to return the writing instrument to the seller against full reimbursement of all costs. If the seller refuses to take it back, three routes remain: instructing a solicitor to enforce the claims for damages, involving the police, and informing the proprietor of the trade mark rights, that is, the manufacturer.
Caution is called for here. Anyone who proceeds against a seller without being absolutely certain that the item is a fake risks an unjustified interference with that seller's business operations (Eingriff in den Gewerbebetrieb) and may become liable for damages himself. Before any further step, the question of authenticity should therefore be settled beyond doubt, in case of uncertainty by the authorised specialist trade.
For the same reason, merely notifying the platform operator that a seller is offering a counterfeit rarely leads anywhere; it is usually followed by no reply at all, or only by an automatically generated standard response. In the case of commercial vendors of counterfeits, it is more effective to approach the trade mark proprietor directly.