I believe the majority of web based sellers are genuine but some auctioneers are suspect.
Just another tip or two to add to the info. All inexperienced buyers should type "fake Mont Blanc pens " into Google search and have a good read.
If the pictures being used are blurred ask the seller for more detailed photographs of - The serial number, the Logo star on the cap, the 925 stamp or other stampings if it is silver, the pen in general.
If the advert picture is this blurred can you be sure?
I think this is more reassuring.
The right logo is less bright white and it has a flat annulus in the silver around the logo, the left is genuine.
You could get a suspect silver cap on a genuine body from a pen originally with a far less valuable resin cap (even the clip with its tell tale PIX symbol on the back of the clip can be transferred to add to the illusion). If its a silver Doue cap pen the bands on the body will be gold not silver.
Showing all the packaging is not good proof that the item is genuine.
Always ask the seller to confirm in an email its genuine and why they believe its genuine, if you buy it and you are suspicious have it looked at by someone who knows Mont Blank pens. Then you have proof of fraud if it is not genuine. Genuine sellers will not be afraid to give you all the evidence you need in my experience.
Happy hunting, there are some super bargain pens to be had.