First, make sure it's not just another domain appraisal scam.
And if it's a legitimate offer, tell them that their current offer isn't close to your expectations and that if they are serious in buying the domain, they need to make their best offer. You can hint them by giving them a range of your pricing (low-mid four figures, five figures whatever).
Read around to learn how you can negotiate with the buyer to raise offers.
Good luck,