Patients are illegally selling NHS-prescribed medications online and pocketing the cash.
A BBC investigation has uncovered people selling unwanted or unused drugs, which can only be prescribed by specialists.
Prescription medicines to treat a variety of ailments were readily available for sale on eBay.
An eBay spokesperson confirmed it had removed a number of listings for sale from the site.
An undercover reporter for BBC's Inside Out West Midlands was illegally sold Caverject, a prescription medication for erectile dysfunction, and Enbrel, used to treat arthritis.
The patient selling Caverject confirmed he received the drugs for free on prescription as he had Type 1 diabetes, but said he "can't use them anymore" and he had "three or four" regular customers.
He said he was prescribed four boxes a month - at a cost of about £500 a year to the NHS. The boxes were listed for sale for £6 each.
The law and prescription-only medicines
- The NHS Choices website states that The Medicines Act 1968 defines three legal categories of medicines - general sales list medicines, pharmacy medicines and prescription-only medicines
- Under this act, most medicines can only be sold or supplied against a prescription at a pharmacy under the supervision of a pharmacist
- Some general sales list medicines can be sold at other premises, such as convenience stores, as long as they are pre-packed and the premises can be closed to exclude the public
- Prescription-only medicines cannot be obtained without a prescription, usually from a GP
- In some cases, medicine may be prescribed by a dentist, nurse, pharmacist, midwife or other healthcare professional, such as an optometrist
- Some prescription-only medicines are classed as controlled medicines, such as morphine, pethidine and methadone, because of their potential for abuse. Stricter legal controls apply to these medicines
- NHS fraud can result in custodial sentences imposed by the courts
- Source: NHS Choices website
Another man was filmed selling Enbrel, a specialist arthritis drug, for £250. The drug costs the NHS about £10,000 a year.
He told the reporter: "This is £800 at the pharmacy."
He said he was prescribed a box every four weeks and sold on any that was unused.
"I'm accumulating a box every three months," he said. "I've only put it up on eBay because the person that was normally collecting from London stopped."
He admitted: "I was thinking [whether] someone's going to get me done for this, because I shouldn't really post an ad like this."
A total of 1.084 billion NHS prescriptions were dispensed in the community in England in 2015, at a cost of £9.267bn. This does not include prescription items dispensed in hospitals, or for the UK as a whole.
Analysis: Jonathan Gibson, Reporter, BBC Inside Out
It's difficult to gauge the extent of this black market trade in NHS prescriptions.
NHS Protect - the dedicated crime unit - couldn't provide an estimate.
But given the ease with which I was able to find and purchase prescriptions from patients, the anecdotal evidence points to it being widespread.
The true scale of the problem of patients selling prescription drugs online is not clear.
The Medicines and Healthcare products Regulatory Agency (MHRA) is responsible for ensuring the supply chain for medicines, medical devices and blood components is safe and secure.
A spokesperson said: "Medicines are not ordinary consumer goods - and appropriately, strict legal controls apply to their sale and supply.
"Prescription medicines are "prescription only" for good reason - and should only be prescribed by doctors or approved healthcare professionals, based on clinical judgement, to ensure patients are getting the right treatment and monitor progress or deal with adverse reactions.
"Selling medicines, other than through a legitimate supply chain, is both illegal and poses unquestionable risks to the patient.
"Medicines have the ability to harm as well as cure - use them as directed by a healthcare professional."
When approached by the BBC for a comment, the wife of the man selling Caverject said they knew what they were doing was wrong but were "not making a lot of money" from it.
Meanwhile, the man who sold the BBC Enbrel said: "I just want to apologise because I know what I've done was wrong."
Asked if he would be selling NHS prescriptions again, he added: "No, 100%."
A statement from eBay said any substance or item that required a prescription from, or the supervision of, a licensed practitioner to dispense was not allowed on the site, but some medicines on the Medicines and Healthcare Products Regulatory Agency General Sales List were allowed.
A spokesperson said: "EBay is a marketplace and sellers must comply with the law. EBay sellers are prohibited from listing prescription drugs, in line with rules set by the Medicines and Healthcare Products Regulatory Agency.
"In addition to our own filters and other technology, eBay works closely with the MHRA and other regulators across the globe who have a direct line to eBay and enable us to remove any listings of concern as soon as they are reported."
You can see this story in full on BBC Inside Out West Midlands at 19:30 GMT on BBC One on Monday 16 January or via iPlayer afterwards.