In this case it seems the customers are unlikely to revolt. Most won't even notice, right?

Great point. For now, it is limited to AZ and NV and for wholesale purchases only. But it seems like a conflict of interest that's hard to prove isn't being abused.

Isn't Amazon's marketplace similar in that regard? They compete with other sellers using Amazon's platform all the time.

Ethically, it seems a bit fuzzy for me given that one of Zillow's big features is there zEstimate. How can they demonstrably prove they aren't tweaking that somehow for their own gain?

I'd argue that Amazon's customers are the buyers of the products, not the sellers using Amazon's platform.

I'd argue that Amazon has many different groups of customers.

Why? BlackRock offers solutions to other asset management firms to make investment decisions. They use the same solution in their asset management division.

Isn't there a firewall between the units?

Zillow has captured the user base so effectively that the customers will have no choice but to continue paying.

See also: Amazon

Every big company does this in some way. The only smart move in business is to be where the money is.