''If we take the very extreme view, people in desperate circumstances might be prepared to martyr themselves, selling their hearts to save their families,'' said Ross Taylor, director of transplant surgery at the Royal Victoria Infirmary in Newcastle Upon Tyne and president of the British Transplantation Society. Mr. Taylor also criticized those tempted to sell their organs for frivolous reasons. ''I have met people prepared to sell their kidneys to buy Porsches or to take a girlfriend on a holiday.''