I fail to see the difference honestly. Just like a new car, the moment you leave GC with a new piece of gear, the value of that gear drops drastically. Why should anyone pay 90% of a new price for a piece of gear second hand without warranty or any guarentee that it wasn't F'ed with in some major way that you won't find out about until way later(truss rod issues, electronics, etc). When I buy a used piece of gear I think about all factors that go into it, including what I'd be able to sell it for down the road if I wanted to. You can call that chronic "lowballing" I guess, but some of us have limited gear resources and have to anticipate what something can really fetch(remember, a lot of musicians are broke cheapskates)
I'm sorry but your "mint" MIA P bass that you got off MF 46 days ago is not worth $1200. I can't even get a bite at my EBMM stingray that I have listed in my towns local classifieds, listed at $800. A few years ago I paid almost 1400 for it. I'm not crying about the "lowballers" who offer me 500 for it. Fortunately I'm not super desperate to sell it that low, but I'm ready to take a hit on it and not get overly butthurt.