Speaking solely for myself, I always try to price items fairly -- to me, and to the prospective buyer. I deliberately do not inflate my asking price simply to leave a lot of "haggle room". Hence, I'd define a low-ball offer as one that is more than 15% below the price that I have asked. I usually succeed in selling most items for my asking price, or maybe 10% less.
I have had only one recent instance of a real "low-ball" offer. I was selling some Kimber 8TC cable at slightly less than half of the MSRP, and got an offer from a guy to buy the cable for 25% of MSRP (about half of what I was asking). I simply sent him a "thanks for inquiring" note, commenting that my ad stated the price as "firm".
I have had only one recent instance of a real "low-ball" offer. I was selling some Kimber 8TC cable at slightly less than half of the MSRP, and got an offer from a guy to buy the cable for 25% of MSRP (about half of what I was asking). I simply sent him a "thanks for inquiring" note, commenting that my ad stated the price as "firm".