Among buyers who make low-ball offers, they may be different. Some buyers look at the comps and mistakenly think your house is worth the same. Other buyers know the value of a house but try to bargain from the basement price.
For the first kind of buyers, there is really nothing you can do, except for a friendly note through their agent that his/her offer price is reasonable if the house is in a poor condition like those of foreclosed homes. They will realize their mistake after a while.
For the 2nd kind of buyers, what I would recommend is to give a counter offer with a price close to your asking price. This will give them a chance to understand that you know the value of a home and will not accept low ball offers so that they can make a 2nd offer if they are serious.
