Buying iPhone from Amazon
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.