I am offering the following tips ( which I hope are useful ) and would welcome any additions on how to spot fake fountain pens (Mont Blancs, Parkers, Pelikans, etc).
1. Ask for a picture of the underside of the nib section. A genuine Mont Blanc's black section will be aligned perfectly with the ink aperture between the tines ( a genuine article is accurately produced down to the smallest detail ). If the black section is not aligned and is either side of the aperture, then it is likely to be a fake.
2. The nib is the most difficult aspect to fake. Always ask for a close up picture of the nib section - iridium tip and 'made in germany' stamped on the nib is a dead give away. To my knowledge, most fountain pen manufacturers do not stamp 'iridium tip' on their nibs. The exception to this is the Genuine Mont Blanc Starwalker pen - it does have the words 'iridium' stamped on the nib section, is nearly always silver ( hence, this pen is faked more than any other ). Always ask for a pic of the floating star on the cap, if it appears to be irregular or not dead centre, it is likely to be fake. The best tip I can give is go to a genuine dealer and actually take a picture of a starwalker ( barrel and cap ). That way you know what to look for.
3. Before you bid, get an emailed assurance from the seller that it is a genuine fountain pen - because if turns out that it isn't , you can ask for your money back or get ebay involved. If you get a vague response to your question - don't bid.
4. Another dead give away, ask the seller if he has more than one item and if you can purchase more than four. A faker always buys in bulk and then sells them individually. Many genuine fountain pens (especially of Limited Edition series) are rather expensive, and if a seller has more than four - it's likely that he has bought in bulk.
5. Hold the pen up to the light. The black plastic should be somewhat transparent.
Montblanc's pens are made of resin (except for the solitaire line) and this natural material is able to hold an electric charge (much like amber). This material also feels more animate and doesn't feel cold to the touch.
6. Look under the clip, and you can read Pix. Thats always a good first look to see if it is fake. A fake will not have it.
7. Counterfeit Limited Edition pens won't usually be in their original special boxes or cases, instead they might come in an ordinary Montblanc box that someone has sold on eBay
8. When buying on eBay, always send seller email with a few questions before placing bid. Also pay attention to feedback ratio and number. So, a seller with over 20 positive feedback score and no negatives deserves trust.
9. On the fountain pens, the number "4810" is on the nib, representing Mont Blanc's height in meters. Mont Blancs all have serial numbers, which is how one can distinguish a fake from the real fountain pen.