^^^ While some sellers may be relatively inflexible, often buyers send offers of 50% or less than the listing price, which perhaps may work for some--as I've seen many sellers list prices that are inflated that much, i.e. by twice what the market price is, so it looks "good" when they accept 50% of that--but for those who list at a fair price, asking them to take 50% of that is a bit much.
ETA: I just declined an offer for 50% of my listing price, as that is way below the cost of the item (it's NIB), and with ebay & paypal fees, I would lose even more. This is for an item which is rare and no longer available retail. My listing price is set at a fair price, so 50% is pretty much an affront. I am not a professional seller, only an occasional one, and I will accept reasonable best offers, but I'd rather donate to charity than to lose money selling to buyers who offer what they know is an unfair price. I buy more than sell, and I would not think of offering lower than value prices to sellers. I do my research so I know what the going prices are for things.