> An owner can list their apartment/house directly online.

How will anyone find the house? If I use an online estate agent, then that's still a middle man. If I publish adverts on Facebook or Google, that's a middle man. If I'm hoping that I can generate enough SEO for my house to appear at the top of searches, that's also relying upon a middle man - the search engine. I guess I could just put a board outside the house with a URL on it and hope someone stops to take a photo.

Estate agents provide that marketing service as well as others around arranging viewings and interaction with solicitors, although that might be UK specific. But they do provide a service that would take a crazy amount of time for you to replicate by yourself for a one-off house sale.