When you receive an offer for one of your domains, you are likely to make a counter offer in order to get the best possible price. They will reply with another offer and so on.
The hard part is not to make the counter offer, but to decide when to stop talking.
Domain name negociations can last months as the seller is usually in no rush at all to sell: after all it costs only a few dollars each year to keep the domain.
But some potential buyers do not see it that way and act on a impulsive act. They want the domain now.
If you wait too long, they will lose interest and you will lose the sale.