I used to sell the more valuable parts (GPU, CPU, Mobo, RAM) fairly religiously when upgrading but haven't done much of that lately.
My son gets my hand-me downs now but even doing that I still have a Ryzen 1600 / 32GB / HD7950 / 27" 1440p 165hz setup just sat doing nothing. I also don't really make use of the monitors I have.
In the kitchen I have my old 3570k / 12GB system setup for streaming video etc.
I then have boxes of various old bits lying around. i5-750, a couple of rubbish prebuilt PCs that relatives were getting rid of, half a dozen mechanical HDs, even got some proper oldschool hardware like printer cables, BNC cable, ISA slot modem, 3dfx Voodoo Graphics card etc.
To be honest if it was easier I would sell more of it but it kind of gets to the stage where I just can't bring myself to let hardware go for super cheap, like say that 3570k/12GB rig, I'd get maybe £50 for it but shipping would cost a fortune, it gets to the point where I'd rather keep hold of stuff as spares.