In most countries, selling organs is illegal. So if you succeed in finding a doctor that would agree to take out your organs, and have a buyer, then both of them are risking several years of prison time. For them, it is much safer and easier to kill you while taking your organs, to prevent you from telling anyone, and of course not having to pay you. Your motive is most likely financial gain. However, when you sell your kidney, you can expect to be paid only a fraction of its worth. For a more detailed answer, you may want to read this Wikipedia article.