We worry so much about our bodies. Western culture has, for example, a very big concern with our hair and its thickness, perhaps because so many of us are prone to lose it (you can check out why your hair is falling out here). At one point or another, we've all had to wonder what the most expensive part of the human body actually is, especially since we, as a species, have sold body parts in our past (and continue to do so!). Whether it's as a joke or to do some real Frankenstein shenanigans, it is a rather interesting subject to discuss. While you generally can't go up to a hospital and offer to sell them your kidney, this doesn't mean your organs are worthless; just that we'll need to look to a different source to find what we want.
Just about every part of the human body can be sold for some amount of money, though exactly how much will depend on who's buying and where you are. Given that every country besides Iran has outlawed organ trade, there aren't very many "official" sources to turn to for pricing, either. Havocscope and others use a mix of government, academic, media, and anonymous sources to get their price figures, so keep that in mind going forward.
Finance Degree Center has a great resource on the "Red Market" of human organ trade, and websites like Havocscope have up to date information on all sorts of illegal dealings including black market organs. Between the two, we should be able to find a mostly definitive answer as to which part of the body would bring in the most cash.
Your first candidates for the most valuable human body parts probably turn to things like the heart or lungs; you know, stuff that you kind of need to live. While these both fetch a pretty high price on the black market ($119,000 and $561,200, respectively), these are not the most valuable organs.
Blood is also pretty valuable. What most of us give away freely to greedy vampires running the blood banks is actually worth around $350 a pint. This trade is lucrative enough that entire illegal blood farming operations have been set up.
Of special note is the brain, which fetches a surprisingly small amount of money illegally, with one report in 2015 pegging brains at about $100 each (probably because brain transplants are already a hard and messy thing to do, with no reported successes in history so far). And what about a gold dental crown? That has gold in it right? Well, if you’re wanting to know, the average payment for per gold crown is around $11-$12.
In the end, though, it comes down to bones. Bone marrow, specifically. Marrow is allegedly worth $23,000 for every gram of it you sell. With around 2,600 grams of marrow in the average human body, that means you've got almost $60 million housed inside your bones right now.
But, let’s not go about selling pieces of ourselves. Think of this as a morbid exercise in quantifying of things we shouldn’t really seek to quantify. Keep your bones and brains, people. If you’re dying to give a part of yourself to the world, think of giving some love instead.