Nothing to do with negotiation… people are just cheap. plan and simply. If I post a part up for $10, someone will offer $5. So people price things a few dollars higher simply because no matter what they will get low balled.
Personally, I started giving all my crap away to all the local guys as I’d rather help them out vs deal with people wanting shit for nothing anyway. And anything of lower value I just throw right into the trash because its a waste of time to even post stuff up for $5.