A great moral question that many bioethicists have contemplated, myself included. Here is clash of market/consumer morality, which can be nearly anything goes in the sense that if there is demand, the markets wants to meet (the sex trades are a great example) and moral thinking, which is the theories of human virtue/flourishing, consequentialist goods, obligations, human rights, and respect for persons that guide our decision making process.
We set limits for human behaviors and argue that we "ought" do such and such and prohibit actions we argue damage/harm human beings ability to live, thrive, and have dignity. Laws reflect the moral framework (the moral theory). Are there things money can't buy? Some argue that morality will always and should always trump markets and consumer desires for the greater good. But as we know that has historically been a feat unless the society has taken on a more morally enlightened view and that exceptionally rare to observe. The chief argument against trading in human organs is the severe commodification of human beings, which is an idea many people religious and secular have worked to keep alive in the age of consumerism. It is a minority perspective, but a strong one. The sex trade, for example, is illegal in many counties and is child pornography, yet there are market demands for both. We forced prostitution and child abuse as severely harmful and exploitative; thus many see the prohibition as justified. The paralleled argument is raised about organ trading and the high potential for abuse and truly reducing humans to commodities to be bought and sold on the market.