So.... this already goes on, Landlords "fish" their properties on the market when tenants move out early, trying to look for higher bidders. If a service like Rentberry was widely adopted, the landlords could also be persuaded to take lower rent bids and rent more often instead of holding out for higher prices. The real win here is for landlords who don't need the "management services" of real-estate companies, so they can get market exposure without giving a big slice to agents.
Freedom of information is good, the guy running Rentberry may be a sensationalist jerk, but the essential concept of an open marketplace for rental price negotiation should serve both sides of the rental agreement. When I rented in Houston, I would have paid an extra 10% to a landlord who would give me a "tenant friendly lease," but there simply were nonesuch on the market, they were all locked up in these super-landlord slanted agreements perpetuated by the real-estate companies. An open marketplace could have made that more available.