I would think the limited availability is because when buying high end gear there are a lot of possible issues. If you can afford this stuff, you are not going to be selling it yourself. I trade in gear when I buy new, I’m not going to be fiddling around. advertising, maybe having people I don’t know come over, worry about shipping and insuring.
Places like Echo and many dealers sell used stuff. They verify it is operating correctly and deal with logistics. No way I am going to buy a $50K original cost piece of audio equipment from I private person I don’t know.

