By Brad Edwards

(CBS) – A black market for medical supplies that diabetics desperately need is raising concerns among federal regulators and doctors.

The U.S. has nearly 30 million diabetics. Those patients use diabetes test strips to monitor blood-sugar levels, a key step in managing the chronic disease.

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But at $2 apiece, the strips are too costly for many patients.

That’s where the black market comes in.

2 Investigator Brad Edwards discovered across the Chicago area, there are numerous people buying and selling strips on street corners and parking lots.

The sellers are typically people who are not testing enough and selling unused strips for extra cash. The average patient will test three times a day.

Or, in some cases, patients have federal health insurance, either Medicare or Medicare, and are receiving more than strips than they need.

A 2013 federal audit identified $425 million worth of questionable Medicare test-strip spending.

As inspector general of the Illinois Department of Healthcare & Family Services, Bradley Hart oversees the state’s $20 billion Medicaid spending.

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Of the black market for test strips, he says: “It’s somebody finding a way to get something free out of the system. The recipient is looking for cash.”

2 Investigators found middlemen advertising on Craigslist, Facebook, and on street signs. They offer up to $20 for an unopened box of 50 name-brand strips. That same box retails for more than $100 at a big-box drugstore.

One middleman said he sold the strips back to a pharmacy.

Another says she was buying them for a doctor.

What they’re doing is not expressly illegal. But federal regulators says this underground economy is costing taxpayers money.

Ideally, patients shouldn’t have extra strips to sell.

“This smells, to me, like a fraud,” says Renato Mariotti, a former federal prosecutor, now in private practice.

CBS 2 asked local doctors and the U.S. Food and Drug Administration about this black market.

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They warn that it’s dangerous to use strips that aren’t handled or stored correctly.

Brad Edwards