Amazon's verified purchase is ridiculously easy to game - a competitor of mine has 10 - 20 accounts shilling all over that store with verified purchases... once you consider the 70% you get back you're only paying $0.30 or w/e to verify the purchase, or nothing if you just set your app free then revert it to paid which will give you a 2 - 3 hour window. If someone from Amazon is reading I'd love to ping you with names!

What I look for is a solid mix of 2/3/4 ratings, it's difficult to fake that, on the Play store you can balance the # of downloads against the # of reviews too and avoid stuff where 30 - 100 percent of 'downloaders' leave 5 star reviews.