On a recent weeknight, Dahlia and Adam Brown came home to a spacious Colonial on a quiet cul-de-sac in Marietta, Georgia. 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 are more introverted, making the idea of prospective buyers tramping through their private space seem excruciating. Across town, Martha Seay...