When a seller is selling a large amount of Montblanc pens, chances are they are all fakes. Generally, since they are quite high end and expensive, legitimate sellers only deal in small amounts. Sellers of the fake versions like to buy in bulk amounts and then sell one at a time. A good rule is to be suspicious of the pens if a seller has more than four to sell since this means he probably purchased in bulk.
Another good way to spot fake Montblanc pens is to examine the pen's nib. True Montblanc pens do not have Iridium Point Germany nibs, so if there is an indicator that the pen has one of these it is a fake. This indicator will be marked on the nib.
Pre-1991 Montblancs do not use serial numbers; if there are no serial numbers, you probably are looking at a genuine pen. When serial numbers are present, look to see if a seller's pens all have the same one. This means that they are most likely fakes. Fakes always have serial numbers but true Montblancs often have them, too. Sometimes even the real ones have the same number because the company does reuse them. When serial numbers are on the pen, remember the other ways of spotting a fake to be on the safe side in your purchase.
Montblanc pens are composed of resin. Pick up the pen and feel it. Resin is essentially plastic so the pen should not feel like metal in your hand. If it feels like metal, that definitely means you are holding a fake. Real dealers are likely to be careful about their wares and not let everyone handle them. If you do a personal examination it may be allowable because you are in a place that sells fakes regularly.