There is a large market for unauthorized online sales of secondhand continuous positive airway pressure (CPAP) devices, which treat obstructive sleep apnea, according to a new study.

Purchased from an authorized vendor through a sleep clinic, a CPAP machine can cost $600 to $2,000 for patients with little or no insurance coverage, the authors write.

“We did not speak directly with sellers or buyers, so we can only speculate on why this market exists, but we suspect that sellers have CPAP machines they no longer use or no longer need, while buyers are unable or unwilling to pay for CPAP through usual methods,” said coauthor Dr. Ken M. Kunisaki of Minneapolis Veterans Affairs Health Care System in Minnesota.

In obstructive sleep apnea, the throat muscles relax and block the airway off and on during sleep. The CPAP machine increases air pressure in the throat to prevent the airway from collapsing.

A secondhand CPAP machine may not cause direct medical harm, but patients may be spending their hard-earned cash on a device that is not properly set up for them, without education on proper use, cleaning procedures, troubleshooting, warranty claims, and manufacturer recalls, Kunisaki said.

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In October of 2014, the researchers searched Craigslist weekly in 18 U.S. cities and regions including Maine, Detroit and San Francisco for CPAP devices. Then in May of 2015 they placed an ad to anonymously survey CPAP buyers in each region.

There were 270 advertisements for secondhand CPAP devices, with more available in larger cities.

“We only looked at the posted advertisements, so we do not know how many of these resulted in a sale,” Kunisaki told Reuters Health by email. “We have not seen any data regarding what percentage of CPAP users are using a secondhand device acquired through one of these online markets.”

More than three quarters of ads did not say who had used the device, why it was being sold or its pressure setting. More than half advertised a mask included with the device without information on the age of the mask or how it had been sterilized.

On average, the devices were listed for $291 and most were $500 or less. Only five of the 270 ads mentioned a prescription requirement, as reported in JAMA Internal Medicine.

“Although it is technically illegal to issue a CPAP machine without a prescription, we are not aware of any lawsuits filed against an individual selling their secondhand CPAP device,” Kunisaki said. “We did not speak directly to any sellers, but we suspect most are unaware of the legal requirement for a prescription.”

The researchers’ ads to survey CPAP buyers were flagged as inappropriate posts on Craigslist and removed within 48 hours.

Reputable companies only sell CPAP machines to patients with a prescription, said Joshua Fogel, professor in the department of business management at the Murray Koppelman School of Business at Brooklyn College. For specialized machines, this can cost as much as $3,500, Fogel told Reuters Health by phone.

Secondhand sellers may be selling their machine because it is defective, and would not disclose that information, said Fogel, who was not part of the new research.

“Ideally, (patients) should get a CPAP machine through their doctor's office or durable medical equipment (DME) company, who will then provide education and ongoing support for the CPAP device and supplies,” he said.

Patients with little or no health insurance coverage may use programs like the American Sleep Apnea Association's CPAP Assistance Program, a better alternative to consumer-to-consumer sales, he said.