On a warm spring evening in New York, dozens of people gathered on a rooftop in Midtown Manhattan to sip fruity cocktails and chat. Shortly after the happy hour began, a woman stepped away from the crowd and went to work.Standing between a backdrop of fake greenery and an iPhone attached to a ring light, she put on an auctioneer’s voice and implored her audience to buy a used sweater.“Let’s get this to $67, you guys,” Iva Lazovic said, smiling and stepping toward the camera. “This is so cute. It’s Lululemon. You are never getting it lower than this at the store. Let’s be real. Posh has the steals and deals.”Ms. Lazovic was one of several women at the event who hopped in f …
