You've just received an offer to purchase your home ... and it's low ... really low. At least it feels really low to you, the seller. Now what?
1) Don't take it personally and respond negatively.
While it may be really difficult to keep your emotions in check, there's no point in getting insulted at a lower offer than you expect or feel isn't fair for your home. It has been a sellers' market for a few years now but the tide has turned and buyers are in the drivers seat. Any offer you receive is an opportunity to move on with your plans.
2) Check again for comparables in your area of homes that are similiar to yours.
Make sure this offer is actually a "low ball" offer. If you've been on the market for a while, there may have been lower sales since your first set your price. Once you're reacquainted yourself with what current prices and conditiions are, you're in a better negotiating position.
3) Assess the offer from all sides.
Is the buyer's agent a seasoned one or inexperienced? Is the buyer's agent from out of the area and not aware of your property's value in neighbourhood, school catchment, etc.? Is the buyer an investor to whom this is strictly a numbers game. (If so, they're likely not going to counter and will just move on) Are there other factors beyond strictly price that you can work with to be more favourable for you. For example, moving completion dates up if you've already bought elsewhere or extending them so you won't have to rent while you look for a home and move twice.
4) Respond with what you'd have used had the offer been acceptable.
There's nothing to be gained by refusing to talk and quite possibly, something to gain. The buyer may have just been "trying the price" in case you were desperate. By responding in a matter of fact manner, you're establishing that you're knowledgeable and willing to negotiate reasonably. Mention a few relative comparables and features that make your home stand out compared to others. Be prepared to go back and forth a few times and above all, be patient.