NEW YORK (CBSNewYork) — From social media to the streets, there seems to be a huge demand for diabetic testing strips, which test glucose levels in the blood.

For people living with diabetes, like Anna DeFeo, a supply of the strips is essential.

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“I’m always testing during the course of the day,” she said.

“If the blood sugar was high, they would know to take extra insulin. If the blood sugar was too low, they would know that they need to take in some sugar,” explained Dr. Robert Gabbay, with the Joslin Diabetes Center.

Because the little strips are so smart, they’re expensive.

“That’s a real challenge,” Dr. Gabbay said.

A box of 50, for example, costs about $80. If you’re under-insured or have no coverage at all, the strips can become a financial burden.

As a result, second-hand strip dealers have set up shop online, placing ads looking to buy unused and unwanted strips.

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CBS2’s John Dias met up with one such dealer in south Jersey.

“He took three boxes, gave me $15 for each box for a total of $45,” Dias reported.

While the dealer claimed to donate the strips with the help of a local church group to people in Africa, experts say they’re more likely to end up online in the new underground economy for diabetic supplies, like cheap test strips.

“Those strips may not be as accurate, and therefore people are making decisions about taking insulin based on bad information,” said Dr. Gabbay.

Why take the chance, he said, on a strip that could  be expired or tampered with?

“You don’t really know what you’re getting… that could potentially affect your health,” he said.

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While buying and selling diabetic test strips is not illegal, it is against the law to deal strips paid for by Medicare or Medicaid. Still, experts say diabetics receiving free or low-cost strips are the primary supplier to second-hand dealers.