Not what I mean. There are several services that provide materials and kits for doing such a sale; Zillow could let the owner control the sale, but provide services to the owner rather than sitting in the middle.
Ah, gotcha. I misunderstood what you meant. You can definitely offer limited services and advertising. You just have to be very careful not to engage in any activities the state might construe as creating an agency relationship, or that are specifically reserved for licensed agents.