  This difference between kidneys and blood reveals a general structural difficulty for altruism arguments against sale. For altruism arguments (insofar as they work at all) work better for those things which are already freely donated on a large scale, than for those things which are hardly freely donated at all. Hence, they will tend to be most successful where, in a sense, they are needed least—because if there is already widespread free donation, then commercialization will be unnecessary. This is not a decisive objection, since there are things which are in short supply in spite of widespread free donation (blood and sperm might be examples of this).  But it is a problem for this style of argument because there will be a tendency for it to be least successful where it is most needed (Radcliffe Richards 1996, 2009, 2012; Wilkinson &amp; Moore 1999).