If you want to sell one of your kidneys then you should be allowed to. It is true what megj681 says, it would increase the availability of organs. Unfortunately, this could potentially lead to two drawbacks that would need to be immediately addressed to avoid them.
One, it could lead to privatization that could easily be corrupt and counter-productive. This could be easily resolved by mandating it to be a government-only business. This will still leave problem number two: where will this money come from to pay the sellers? The government probably doesn't have money laying around to set aside to pay people who decide to sell tissue and organs. This would have to be solved before moving forward, but I do think the idea is good.
It's common knowledge that people, when provided with motivation or an incentive to do something, are more likely to do it. And this is especially true with money as an incentive. Daily, children and adults are dying because of lack of organs, and legalizing the sale of organs will put an end to this. By increasing the amount of organs in the market, more people whose lives are in danger will be able to live longer and happier.
Now the drug cartels can move up here, and legally harvest Organs from " Willing " subjects? Also, this will tear up the desperately poor who will sell their organs just so they can feed themselves. There are better ways to get Organs now adays. Stem Cell technology, we should fund that instead of destroying people's bodies.
Capitalising on human organs is frankly morally wrong. It's a slippery slope and will only lead to a more entrenched, horrifying divide between rich and poor, with the rich getting the 'premium' healthy organs and the poor getting the defective organs if any. With regards to selling organs being an 'incentive' to donate them, money shouldn't selfishly be the motivation for one person to save another's life, it should be human compassion and willingness to help others. - inb4 'that's idealistic' because it happens anyway, why else do a number of people already carry organ donor cards. If we want to increase the number of human organs available, donation should be an opt-out process.
The profiting of human organs should never be allowed ever! If human organ sales became legal, it would only benefit the rich and it will only hurt the poor. Only the poor, homeless, and destitute would be selling and only the rich would be buying. This leads to endless opportunities for corruption and exploitation.