Gnux, if the phone works, I assure you it's a real N900 unless it's obviously running Symbian or some knock-off OS, or obviously looks different from the pictures you've seen of N900s in other ways.

Just because the seller sells crap accessories with unknown condition phones doesn't mean the phones are fake - they might be, and early on when the N900 first came out there was at least one somewhat convincing-looking N900 immitation out there, but I don't think this guy's selling fake N900s. (Actually, I think this guy would if he had some, judging by his utter willingness to scam people - but I think it'd be harder to buy a fake N900 nowadays than a real one [because most people who bought them probably just threw them out or otherwise didn't bother reselling them.)