It seems like it should be reasonable to have a review warning people reading them that many of the 5 star reviews are suspect as the reviewer was compensated by the seller.
No, because amazon is a marketplace, for example, a product can have more than one seller. because that the review is about the product and not about the seller.
The right thing to do is complain about the seller on feedback and amazon delete all reviews from this specific seller buyers.
Wouldn't this kind of thing generally be coming straight from the manufacturer, or at least a fairly dominant seller for the product? Since they're paying for 5-star reviews of the product.
Could be, heh. I'm using amazon brazil, and here amazon isn't tooooo strong like USA. here in Brazil amazon works with other sellers in order to expand, but in a near future I really believe amazon will works like in USA.