1) As far as I know genuine N900 is designed in Finland but made in Korea (South Korea to be more specific).
2) As far as I know of South Korea was the only place where real N900 was produced, they have different hardware revisions but as far as I know of for production based N900 were all in South Korea.
3) The easiest way to know if the seller is actually selling a genuine N900 is to compare the pictures the seller has with your real N900. Fake N900 may sport:

• Weird designs like built-in dual SIM card slots
• Weird underside of the backside cover where there was meant to be two rectangle stickers, one is coloured white and the other is coloured blue. If you're interested, that sticker or plastic bit is for sensor to pick up whether or not the lens door is actually open or not.
• Antenna sticking out of where stylus is meant to be in.
• TV capability
• Does not look like the real N900 (you can check pictures against wikipedia website)
• Java phone, N900 does not have Java capability by default.
• Does not list maemo as the operating system or some other operating system.
• Internal memory capacity as listed does not state 32GB.
• The specifications does not match with reviews sites.
• This, does not look like a genuine N900 at all