Pretty sure if the seller turns out to not be legit then they will get you replacement tickets. They hold your money & don't actually pay the seller until after you've gone to the event so there isn't really any motivation for someone to sell fake tickets. No one I know who has used Viagogo has ever been scammed by them.
Just a few words like I've told others - have your tickets sent to your hotel, rather than your house, because they don't get released until a week before the match. And as for seats, almost positive that the actual traveling Juve fans will be in 3° blu, but most sections of the Meazza would be "safe" for away fans, just avoid 2° verde & I wouldn't sit in the tier underneath them either.
Hope that helps, if you have any more questions I can try to answer.