Two days ago, at precisely 4pm, a man from Gloucester called Andy submitted the following post to the message board of an American-based website. "I am a healthy 22-year-old male who wishes to sell a kidney, perhaps some liver tissue. I want more information ie how much are they worth?" Thirty-four minutes later, Tash, also British, wrote: "I am willing to sell a kidney. I am a healthy 27-year-old female. Can anyone help?" Yesterday, apparently not yet having had a response, Andy was back. "I left a notice prior about selling organs I would like offers for partial liver, a kidney or testicle." More than 30 people have contacted the website in the past month offering similar services.

Andy is almost certainly breaking the law. It is illegal in this country to buy, sell, advertise or seek to procure any of the human organs or tissue that can usefully be sliced off one person and sewn onto another - hearts, lungs, kidneys, livers, pancreas, small bowel, corneas, bone marrow, skin, heart valves or tendons. However sick you may be, whatever your income, occupation or celebrity status, it is not possible in this country to jump the queue. You will join the end and wait your turn like everyone else.

Increasingly, however, the wealthy unwell are proving themselves unwilling to hang about - and prepared to ignore the law for the sake of a vital organ. In western Europe, 40,000 people are currently awaiting a kidney transplant; between 15% and 30% of them will die while they are in the queue. Increased demand, reduced supply - it all makes for a buoyant market for those prepared to pay to queue-jump. So Andy hopes, at least.

So how much, exactly, does it cost to buy oneself some choice human offal? That depends what you want, and where you get it from. The going rate for a kidney transplant from eastern Europe, according to a study earlier this year by the Council of Europe, is USD$100-200,000 (£58,000 - £116,000), of which donors, typically young, male Moldovans or Romanians "between 18 and 28 years of age living in poor conditions in rural parts of the country", can expect to see $2,500 (£1,400). That is the top end of the market, however. A survey by the American Medical Association in 2002 found that 305 kidney donors in Chennai, India, were given only $1,070 (£620) each - 96% said they were forced into the donation through poverty. There are reports of young Indians selling kidneys for as little as £100.

The market that has become known as "transplant tourism" developed in the 80s, when wealthy Asians began travelling to India and south-east Asia to buy themselves organs. New supply routes, first from South America, then from south-east Asia and eastern Europe, later developed, many of them catering for specific markets: Middle-Eastern Arabs, for instance, are known to have favoured India. In Israel, where buying organs is so acceptable that medical insurance companies will even partly remunerate patients' costs, the road to Turkey, to be fitted with a Moldovan, Romanian and Estonian kidney, is a well worn one. There have long been allegations of organ harvesting from executed Chinese prisoners; some evidence exists of Chinese links to the US market.

Professor Nadey Hakim, president of the Royal Society of Medicine's transplant committee, says the major supplier for Britons is the Philippines, though other countries, which he is reluctant to name, also offer plentiful fresh meat for export. He is personally aware of "a significant number" of people who have bought kidneys abroad, he says, typically paying "hundreds of thousands of dollars". It is only those who develop complications after they come home, however, who are likely to require his services. "I have heard of some cases where the patient was given the transplant, and shortly afterwards put in a taxi and on a plane and sent home, with the minimum of drugs. We don't know, either, whether they had checked if the donor was compatible, or had a transmissible disease." A survey last year of 12 transplant units around the UK uncovered 29 known cases of patients who had bought kidneys abroad - more than half the organs failed, and a third of the patients died.

Though it is possible to donate a portion of one's liver - and even, in very rare cases, part of a lung - while one is alive, Hakim says the only notable cash-for-organs market is in kidneys. Recent tabloid reports of asylum seekers being monitored to see if they had only one eye, having potentially sold the other to pay for their passage to Britain, are politely but firmly rubbished by the Home Office. (Testicles, offered in great numbers and with no little enthusiasm on the website on which Andy is advertising, are not at present known to offer a useful transplantable function, says a spokeswoman from Transplant UK, which manages the British transplant database).

But such is the sketchiness of the information, it is impossible to rule out a more macabre market in other organs. One French NGO, Terre des Hommes, has estimated that 6,000 children aged between 12 and 16 are trafficked each year into western Europe, mostly from eastern Europe. More than 650 are known to work as child prostitutes in Italy, but the charity has noted that, "the number of children going missing in the east does not tally with the numbers we trace in Europe. We know that gangs offer children for sale dead or alive. We can only conclude that the missing children die or are killed for their organs."

But until more organs become available, it seems likely the market will remain buoyant, and be enthusiastically endorsed by former satisfied customers. In March, Thor Andersen, a London-based property tycoon, paid £25,000 to travel to Pakistan to buy a kidney from a 22-year-old woman. She received a fee of £2,000, plus a further £1,000 "to show my appreciation". "I won't lose any sleep over what I did. The girl needed money and I needed my life back." Instead of becoming outraged at what he had done, Andersen argued later, Britons should automatically be put on to a transplant register of which they could opt out. "Then people wouldn't have to see rich people like me who have gone to a poor country to buy a kidney."