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 at this store on W. 116th Street, each strip is also a lucrative commodity, part of an informal economy in unused strips nationwide. Often the sellers are insured and paid little out of pocket for the strips; the buyers may be underinsured or uninsured, and unable to pay retail prices , which can run well over $100 for a box of 100 strips.

Some clinicians are surprised to learn of this vast resale market, but it has existed for decades, an unusual example of the vagaries of American health care. Unlike the resale of prescription drugs, which is prohibited by law, it is generally legal to resell unused test strips.