The safest course is to buy from an authorised specialist dealer. Anyone who nevertheless buys at auction should bid only on listings in which the seller expressly warrants authenticity. A pointed enquiry before the end of the auction settles the matter quickly: in the case of a counterfeit, an answer usually fails to arrive. Personal contact also helps in forming a judgement of the seller.
Feedback ratings are a further, if imperfect, criterion. A seller who suddenly offers several Montblanc writing instruments or other high-priced articles without any feedback history at all is suspect. It is worth looking at the purchases and sales made so far: if only very cheap articles have been traded up to now, particular caution is called for; if, on the other hand, the seller has sold exclusive writing instruments before, the risk is low. This is no guarantee, for counterfeits are offered by commercial high-volume sellers with good feedback as well.
If doubts remain, one should request detailed photographs before placing a bid, or ask the specialist dealer.