I rarely (not sure I’ve EVER) sold a hard drive. If I’m done with a machine, I’ll typically take the hard drive out and sell, donate, or trash the machine. The hard drive gets the platters removed and shredded. I call it “Still Thinking’s Low Level Format with Extreme Prejudice (tm)”.
Had to laugh at the title you have devised for your techique for dealing with no longer needed hard drives.
I use the same process: full disassembly and deep, heavy scoring of the platters followed by personally taking them to the electronics salvage point at the county dump.
Always get a smile when hand them a large ziplock bag full of hard drive components.