I don't honestly understand why someone would waste their time lowballing like this (my post above was a real exchange between a buyer and I), as it wastes my time and their own. Sometimes when I get a lowball offer I'll just counter with the original price and not even bother being a smartass - anything to force them to click.
What I have noticed is there's very little haggling actually happening with these "BO" deals. What usually seems to happen is, when I get a reasonable offer (within 30% of my list), I'll knock 5% off the price and the buyer, more often than not, accepts. No counter-offers beyond that. If it were me, I'd probably try to erode the seller a bit more.