I hate to break it to you, but I'm 90% sure you got screwed into buying a fake. I've also purchased many designer items on ebay and learned a few tricks to tell fake from real.
1)If they do not explicitly claim it is 100% authentic, be very suspicious, even if they do, you still have to take it with a grain of salt.
2)If they ship from eastern europe or asia (regardless of where their location is listed), this is a red flag that it very well might be fake.
3)If they have low feedback like this guy, be very suspicious (they should have at least 20-50 sales before you're comfortable dealing with them because most other buyers might also get duped, but you want to check the feedback to see if one sharp person out there picked up on the forgery. Sometimes these con artists give refunds to people who leave bad feedback in exchange for removing their feedback, so any withdrawn feedback should be noted.)
4)If they don't show you close up pictures of tags (the tags inside and the paper tags), be very suspicious. Whenever I sell something, I'm always sure to have tons of pictures of the tags.
5)If they're selling items from famous designer names (e.g. gucci, prada, armani, versace, etc.) be suspicious because these are all very juicy targets for fakes. Mont Blanc DEFINETLY falls in this category. I've seen asian fakes of mont blanc goods that look almost as real as the real deal.
When I first looked at the bag, I couldn't tell if it was real or not, but I do find it strange it comes without tags or any claim of authenticity. But after looking at some of his other items for sale, I would not trust him at all. The tag on the dolce & gabbana bag looks nothing like real D&G tags you see in stores, it looks like something he printed on his computer at home. Same with the bally messenger bag, it comes with a red dust cover that doesn't have the right font face and looks totally fake. Even the little bally tag on the side looks off. Little details like that matter.
I'm not saying this to rain on your parade, but I've learned this from personal experience. I've brought burberry jackets and shirts on ebay only to find out they were fake 6 months later when ebay sent me a letter saying the seller was shut down for selling fakes. I've brought items like Salvatore Ferragamo and Fendi belts which I'm 100% sure are real, and then there are a few other items where I'm 99% sure they're real even after comparing them to the real deal in boutiques, but I will never be totally sure.
As much as it is a harsh kick in the pocketbook, if you're not an expert on what you're buying, you should just bite the bullet and pay retail. E-bay is very much a buyer beware marketplace, despite all the efforts by staff to ensure everything is legit.
Also, be cautious when comparing it to other items at a mont blanc boutique. Sales associates are not always knowledgeable enough to tell real from fake. As well, I've heard stories that some especially snobbish sales associates who look down their nose at buying at a discount on the internet would stoop to telling people an item is fake, even if its real in hopes of shaming them into returning the item, and buying the it from them.
For what its worth it is a nice looking briefcase, and if it is fake, it seems to be a pretty good fake. But if I were you, I'd be demanding my money back right now, because that is an insane amount of money for a knock-off. If you're going to buy knock-offs and you live in NYC, you can probably buy 10 fake mont blanc bags for that price on Canal street.
Hope this helps,
Omair