Buying iPhone from Amazon I live in Europe, and here I have to pay 850 Euros for a new iPhone 6s with 64GB of memory. I've checked that on German Amazon the very same model price is 700 Euros. The only concern I have is that previously I have only bought Apple products from Apple Stores, and I wonder whether it is safe to purchase iPhone from Amazon (or let's say, as safe as any other product sold on Amazon), or is it a 100% no?
It depends on the seller. If it is direct from Amazon you should be fine. The big difference would be the return policies. 

If it is from a reseller you want to make sure to check for activation lock https://icloud.com/activationlock and it may not come with a full warranty. Also who knows if there are non-apple parts involved voiding any warranty, or if it is from a different region like Saudi Arabia that doesn't have FaceTime, or Japan that forces camera noise, or any other situations which could make warranty repair very difficult.