Apps like TikTok and Whatnot allow small-business owners to peddle their wares in real time—and make real profits. Aaliyah Arnold, the 21-year-old founder of BossUp Cosmetics, goes live on TikTok a few times a week. Each livestream will last anywhere from 4 to 12 hours. Thousands tune in to watch her pack mystery boxes for customers, give away products, and teach makeup tutorials.