HOW MANY PEOPLE do you know who have scored an authentic Sputnik lamp on eBay for $30? Exactly. When Christiane Lemieux sold her interior-design retail brand DwellStudio to Wayfair—now the largest e-commerce destination for home furnishings and décor—in 2013, she stayed on as executive creative director. Thanks to Wayfair, which she refers to as the “university of e-commerce,” she soon acquired PhD-level authority in digital decorating and shopping. Ms. Lemieux’s strategy for her Sputnik killing: “It was knowing exactly what I wanted, putting in as many search terms as possible and sifting through.”

Having...