The only way you can be sure you are getting a 100% genuine product is to get it delivered to you in the mail with a sealed COA sticker with the product key on it. The product key should not be visible until you break a seal that says you agree to the license. Or order it directly from the manufacturer, in this case Microsoft.

You take your chances with the ads for "product key only and electronic download". You might be OK, or your might get burned. And sometimes you don't get burned until some time in the future.