The number of living kidney donations is increasing in many countries, in response to increasing demand, lengthening waiting lists for transplants from deceased donors, and, in some areas, cultural or religious resistance to deceased donation. In most such donations the donor and recipient are related genetically or emotionally, but there are various routes by which a donor may give a kidney to a recipient who is a stranger. The practice of paired, pooled or chained donation - in which a recipient receives an organ from a stranger, in return for which that recipient‘s emotionally or genetically related (but incompatible) donor gives a kidney to another stranger - is accepted and growing.
In contrast, the selling of organs by donors is controversial and generally illegal, except in a few countries.,
Journal of Psychosomatic Research
Erasmus MC: University Medical Center Rotterdam

Potts, S. (Stephen), Vitinius, F. (Frank), Erim, Y. (Yesim), Gazdag, G. (Gabor), Gribble, R. (Robert), Ismail, S., … Zimbrean, P. (Paula). (2018). Mental health assessment of altruistic non-directed kidney donors: An EAPM consensus statement. Journal of Psychosomatic Research, 107, 26–32. doi:10.1016/j.jpsychores.2017.12.001