I see what you mean and yes, I agree experience and doing a build does add value, but to me it comes to whether or not it's worth it to pay for an item versus just get it myself. In the case of zombie heads, for me personally, why pay $2000 when I can just murderize zombies myself? Zombies are everywhere, including grinders. Bear heads are infinitely harder to attain, elder guardians are limited, ghast heads are a pain in the butt.
I also know how to build a grinder and I have the patience to do so. Some others may not have that skill or patience and may want to pay 2000 for even a relatively common item...so it comes to whether or not the buyer thinks it's worth it. Maybe I have a diamond ring my great grandmother gave me and it's worth no less than a million bucks to me. That's great that I think it's worth it, but no one's ever going to pay that so if I truly want to move that item, I have to lower the price. My perception of it's value isn't going to sell it. And if I can't sell it for what I think it's worth, then it comes to the choice between whether or not I should sell it at all then. Maybe I think if no one is willing to pay a million bucks, well then their loss. And that's fine too.
But again, those are just my thoughts. I know you're not attacking.
I'm not attacking anyone either, just offering my own thought processes.
And of course there's always outliers in the buying process. I bought a nether star for 15k just for the express purpose of decoration - a totally frivolous buy that others would prolly think weird of me for. But I bought it from a friend, because she had given me some extra cash during an exchange. So ya know...it's a complex issue whether or not something sells. I sell arrows for a buck a piece, have for a month or so, and only ever sold 64. *shrugs*