On a recent weeknight, Dahlia and Adam Brown came home to their spacious Colonial on a quiet cul-de-sac in Marietta, Ga. The Browns both work demanding jobs and have two young sons. They bought the house in June using Knock, a company that's trying to revolutionize the real-estate industry with a "home trade-in platform" making it easier to buy and sell at once. That solution was ideal for the Browns, who are just as busy as most couples but more introverted, making the idea of prospective buyers tramping through their private space seem excruciating.

Across town, Martha Seay was overseeing movers in a rambling brown ranch-style house nestled among tall hickory trees. The day before, she had closed on the sale of the house, where she and her husband had raised their family, to the real-estate company Zillow. The next day she would leave for Florida's Gulf Coast, where the couple had just bought a retirement home.