I agree with Necromancer, there are no signs of it being a fake. There's another real one listed in Japan for $500 but it's not sealed.
Sapphire still goes for between $200 - $300 on eBay. I'd rather not pay double for a game already going for hundreds just to have it extra mint and extra complete. But anyone interested in doing so is looking for more than a game to play. At least Sapphire is genuinely rare, but not $500 rare.
I wonder what the seller of the sealed copy would say if you asked them if you could still buy it even though you are definitely going to open it? Probably that they don't really care about preserving a collectible, only that they get paid.