Item 1) Currently, sellers give out coupons codes that make a $50 item into a $5 item. People buy in reviews group buy it...it counts as a purchase and Amazon marks their review as "Verified Purchase".
Item 2) Professional reviewers have purchased multiple products above certain threshold.
Easier way to stop this is to prevent reviews from people who used a coupon to buy something and the price is above some threshold (i.e. seller doesn't knock to $5 between 10PM and midnight for their reviewers to buy it).
If I can think of this solution - why can't Amazon? Simple answer is they know of this behavior and encourage it.
I have shifted my purchasing behavior to buy from Walmart, Target, Ebay when at all possible. I know Walmart isn't exactly an angel - but I'd rather have 2 devils in the marketplace than 1. I even try to buy books from Barnes And Noble...but unfortunately the fulfillment time + price is hard to beat w/ Amazon. I rarely have books above the B&N threshold for free shipping.