There is a very huge difference between a reasonable counter offer and a low ball number. A low ball number is an attempt to get something for nothing and totally not considering market value.
Any potential buyer worth their salt knows how much the blue book value of an item is. Also take into account an item's rarity that that totally changes the blue book value.
I research the current market value (high, average, low, etc.) of any item that I plan on selling, taking also into account condition, having the accessories, original boxes, etc. and place my asking price accordingly. I typically price items to sell so my price really doesn't have my room. Also, I am never desperate to sell.
So, when I receive a low ball number I will completely ignore it. If I receive a reasonable counter, I will consider it.
People watch way too much of these insulting flipper tv shows and think that there is actually a free lunch. I don't have time for insulting flippers or lowballers
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