I knew girl a long time ago who was offered a lot of money for her hair because it was strawberry blonde, waist length and naturally curly but she couldn't part with it.
Obviously SOMEONE is selling the hair, because that's where human hair braiding/weaving hair comes from- although a good majority is Asian (including India) hair, sometimes from cadavars.
My cousin and her daughter donated their butt-length hair to that charity that makes wigs for cancer patients.
I think if my own hair grew easily (it grows nicely, but not without some work), I would sell it; it will grow back. Since it takes more care for me to get some substantial length, I'd have to say no.
There is no WAY I would ever buy hair from a classified, though. Human hair for sell through salons and such have been sterilized and everything possible to make sure there are no "bugs" or fungus or whatever, who knows from a classified what else is riding along with that hair, lol!!