I was recently part of a discussion about whether kidneys should be legally bought and sold in the US. It's worth noting that the discussion usually concerns body parts we can't readily notice rather than eyes, ears, hands, etc. The simple act of changing one word for another "should there be a market for the poor to have their eyes gouged out?" can shed light on the answer. But for the sake of argument, we can pretend the same people who want to buy kidneys have no interest whatsoever in buying anything else.
One of the main problems with allowing organs to be bought and sold is that the seller often can't afford not to sell - it's rarely a voluntary choice. Selling the kidney could mean: the difference between sending a child to a decent school or a poor one; the difference between healthcare or not; the difference between having a car to drive to a better job or staying at an old one; the difference between trying to find space in a homeless shelter or getting an apartment; the difference between eating that week or not. This is a market designed solely to exploit the poorest and most vulnerable members of our community - and it's designed so only the wealthy can benefit. Applying the typical "people behave towards their own self-interest and everyone benefits from voluntary choice" model does not work in a market like this. It produces an efficient market, but it produces a deeply unjust market. Exploiting the most vulnerable members of society should never be respected, and it certainly should never be allowed to masquerade as moral.
All men are created equal. When we take by coercion the bodies of the most vulnerable, we forget this basic tenant of humanity. We turn the most vulnerable into the least equal. When we take the poor and transform their bodies into commodities to be bought and sold, they lose their dignity and we lose our humanity.