In the only country where the organ trade is legal, the streets near hospitals have been turned into a 'kidney eBay'

Marzieh's biggest challenge in life is to come up with money for her daughter's wedding. In Persian custom, it is the parents' duty to provide a dowry, known as jahizieh, and as a widow from north Iran, she feels it is important to fulfil her responsibility and protect the family's honour.

To achieve this, she is ready to sell one of her kidneys. If she is successful, she will travel to one of Tehran's kidney transplant centres and have it removed. She will have to cope with only one kidney from then on, but she will have performed her duties by her daughter.

"It is getting too late for my daughter to marry – her moment has already passed," she said.

Iran is the only country where the selling and buying of kidneys is legal. As a result, there is no shortage of the organs – but for those trying to sell a kidney, there is a lot of competition.

In order to advertise her kidney, Marzieh has written her blood type and her phone number on pieces of paper and has posted them along the street close to several of Tehran's major hospitals, home to the country's major kidney transplant centres.

Others have done the same. Some have written in big letters or in bright colours to attract attention; some have sprayed their information on the walls of public or even private properties.

"Kidney for sale," reads one ad, carrying the donor's blood type, O+, and a mobile number, with a note emphasising "urgent", insinuating that the donor is prepared to consider discounts. Another similar ad reads: "Attention, attention, a healthy kidney for sale, O+." Many are handwritten, though some have typed the ads to make them look better. "24 years old, kidney for sale," another reads. "Tested healthy."

Competition means that some ads have been torn down. Some have added their information to ads by other donors. Others have placed their ads on people's doors or simply written them in marker pen on trees where they think they will catch people's attention.

At the heart of the capital, near the Charity Association for the Support of Kidney Patients (CASKP), the number of ads has made the streets of Tehran into a sort of kidney eBay. "My six-month-old baby was paralysed after falling from the hands of my wife," said Ali, 28, from the northern city of Nur in the Iranian province of Mazandaran. "I have to find 20m rials [around £7,500] for my child's operation." He hopes he will get 12m rials for his kidney.

Iran's controversial kidney procurement system, which has been praised by many experts and criticised by others, allows people to sell and buy kidneys under the state-regulated surveillance of two non-profit organisations, the CASKP and the Charity Foundation for Special Diseases. These charities facilitate the process by finding potential vendors and introducing them to the recipients, and are charged with checking the compatibility of a possible donation and ensuring a fair trade.

After the transplant, the vendor is compensated by both the government and the recipient. In an interview with the semi-official Mehr news agency, the CASKP's director, Mostafa Ghassemi, estimated the total official price list to be around 7m rials, of which 1m is paid by the government. Iranians are not allowed to donate kidneys to non-citizens.

"In 2010, a total of 2,285 kidney transplants took place in the country, of which 1,690 kidneys were supplied from volunteers and 595 from those clinically brain-dead," he said. According to Mehr, the majority of people selling kidneys are aged 20-30. Despite the state control, bureaucracy and time-consuming procedures have left the door open for non-official direct negotiations, making the Iranian system more like a kidney market.

Dr Benjamin Hippen, a transplant nephrologist with the Carolinas medical centre in North Carolina, US, has studied successes, deficiencies and the ambiguities of the Iranian system.

Making a judgment about whether the 20-year-old system as a whole has been successful was complicated, he said. "The majority of those selling kidneys in Iran are disproportionately poor, and information about the long-term outcomes for sellers is quite limited. Too, it is increasingly clear that there are many different systems, rather than a single unified system in Iran.

"That said, Iran appears to have successfully addressed the shortage of organs, incentives for organs have not substantially attenuated the growth and development of organ procurement from deceased donors, and reported outcomes for recipients have been favourable."

Comparing Iran with Pakistan, where organ trafficking is nominally illegal but still occurs, Hippen, who is an associate editor of the American Journal of Transplantation, said: "It seems to me that if Iran had not developed a system of incentives, the situation there today would look very much like the state of affairs in countries such as Pakistan."

In the US, more than 100,000 people were estimated to be on the waiting list for kidney transplants in 2010 – waiting lists were eliminated in Iran in 1999.

Hippen has pointed out that "since 1999, more than 30,000 US patients with kidney failure have died waiting for an organ that never arrived".

Arguing in favour of allowing people to sell their kidneys, Sue Rabbitt Roff, a senior research fellow at the University of Dundee, said last year that it was time to "pilot paid provision of live kidneys in the UK under strict rules of access and equity".