I think you've got the right idea. It's about incentives. With fake positive reviews, the seller has an incentive to leave them up, and Amazon has no particul

I think you've got the right idea. It's about incentives. With fake positive reviews, the seller has an incentive to leave them up, and Amazon has no particular incentive to remove them -- you, a random user, has to somehow convince Amazon to take them down. With fake negative reviews, the seller has an incentive to get them taken down, and has the data to prove that the reviewer didn't actually buy the item. (Actually buying all your competitor's items to leave fake negative reviews about them would dramatically increase the cost of the tactic.) End result: ignore positive reviews because they're easily faked, check negative reviews because they're probably accurate.