It's different when every rating is gated by actual money-changing-hands-in-commerce, as with AirBNB, Uber, and similar.

Further, with the overlay of a giant social network, you can rent your spot out only to people with whom you share some indirect but useful level of acquaintance. So you can ask before renting: "Is Mr. X an inconsiderate ass?" And Mr. X knows bad behavior can affect their reputation moreso than just via a semi-anonymous blurb on a website.

That's not true. We take reputation extremely seriously. Unlike traditional listing sites, every review is tied to a financial transaction. Additionally, we have many other mechanisms in place to prevent astroturfing.

It's hardly rocket science to tell whether a review is astroturfing or not..