When a buyer makes an offer , carefully consider it. Do not get offended if the offer is absolutely low. Getting an offer is positive an shows interest. Start with making a counter offer & Present the buyer with alternatives, payment options, etc. after you have justified the price or ROI.
Assess your buyer's budget
If the buyer says that all he can pay is $xxx and you are asking $yyyy, you can propose to him that there are financing options available like on domaincapital.com and similar sites.
Know the domain selling market
How you judge an offer also can depend on market conditions. Domain market does fluctuate, If the selling market is slow, then you may have to consider appropriate counter offer. In a hot market where multiple offers are likely, set your own terms when countering an offer. Meaning how you want the escrow, when do you want they payment, how much time will it take to transfer the name etc.
Rarely is a first offer the buyer's absolute highest price they are willing to pay. Negotiating is part of the domain selling process.