How to make sure an iPhone 7 has not been tampered with before purchase
Buy your device from a reputable company.
(Apple, BestBuy, AT&amp;T, etc)
That's the only surefire way you are going to know that the phone (or device) you just shelled the equivalent of a half a rent check for a non-ocean view 600 sq. ft. studio in Miami Beach is of the highest quality, reliability and most importantly authenticity.
As always, the old adage always holds true:  If it's too good to be true, it probably is.
Now, the iPhone 7 has been out less than a month since being released and you are buying one through the secondary market, you should be wary.  There may be nothing wrong with it, but given that carriers and major retailers give you 15 days to return for a full refund...you have to wonder why they are selling it already.