Jeenah Moon for The New York Times On most afternoons, people arrive from across New York City with backpacks and plastic bags filled with boxes of small plastic strips, forming a line on the sidewalk outside a Harlem storefront. Hanging from the awning, a banner reads: “Get cash with your extra diabetic test strips.” Each strip is a laminate of plastic and chemicals little bigger than a fingernail, a single-use diagnostic test for measuring blood sugar. More than 30 million Americans have Type 1 and Type 2 diabetes, and most use several test strips daily to monitor their condition. But...