I used to be a Medic and Firefighter-- in short I was an adrenaline junkie. And now I'm old and retired. And my body is bent and currently a bit broken. I seek to learn how to be dull.
Despite the enjoyment of watching the hummingbirds at my feeders and the light gardening I do. I still think I could become even more satisfied with my dullness.
Guide me to complete dullness!
Remember how mad the music industry got when CDs came out, because a CD could change hands several times with no degradation, unlike with LPs and tape cassettes that got worse with every play? And now software is a license; even if you have all the install files, you might not be able to get verified by a server to install it.
Just saying, our late-stage capitalist system is moving away from ownership, and part of that is taking away the right to transfer ownership.
For medical stuff, it's already a closed market. You can't sell anything medical without violaiting a federal law and the marketplace administrators banning your account (I tried unloading some pet medications on a freecycle group once)....
I'd also say that people might be squamish to remove Aunt Gladys' hip; though maybe if funeral homes want to make more money, they can lobby for their right to strip us down for parts.
You sound pretty well positioned to find yourself a used knee, though, I bet you already know where you can find one if you think about it.