According to a study in JAMA Internal Medicine, there is a large market for unauthorized online sales of secondhand CPAP devices, Reuters reports.

“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,” coauthor Dr Ken M. Kunisaki of Minneapolis Veterans Affairs Health Care System in Minnesota told Reuters.

Researchers searched Craigslist weekly in 18 US cities and regions in the month of October 2014. 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.”

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.