Using simple supply and demand analysis, think about the system of allocating human kidneys, we have two kidneys but should be able to live with only one. The law that forbids the sale of kidney's and other human organs, allows their voluntary donation. This means that there is a bigger shortage or excess demand than there otherwise would be. What is your view of the law forbidding the sale of human organs?
What about the sale of blood? Is this different? How so?