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Christ, yeah. Remember the vendors have almost certainly selected the estate agent who said they'd get the highest price, so have already started house hunting for their next place with that figure firmly in their minds. If someone offers way below that, i.e. more than 15-20% of the asking price, then they take offence as their dreams of their new home at the top of their budget, recede into the distance.
Do people take it that personally? If someone offered us that much less I would take it as a negotiating point.