Which parts of a Mac laptop to people buy by the part? I congratulate you for your thriftiness and ecological contribution, but surely this is an extremely niche practice.

Any piece that you can dissemble, from the top case and keyboard, to the logic board, to the display and bezel. And on older ones, they even had those amazing removable drives and RAM... and remember, since the drives at least were using a slightly proprietary form factor, you either have to buy a used OEM drive or a more expensive 3rd party replacement like the ones from OWC.

But even an old broken MacBook Air can get $100+ depending on the specific model, not parted out at all.

I've even sold the set of screws for $10. People would rather buy from me than risk non-OEM ones.

Yep, all the proprietary parts work out well as long as you don't end up needing them yourself.