How we change what others think, feel, believe and do
When they name their price or what they want in exchange for what you are offering, visibly wince.
Look startled and shocked. Look at them in disbelief.
Say nothing, as if you are shocked into silence. And then wait for them to make another offer.
If they say nothing (give them plenty of time), you can ask them to repeat it or ask if they are sure.
I am buying a car on a private sale. The seller names his price. I jump a little take a sharp intake of breath and look alarmed. I take a few paces back from the car and shake my head. Then I look at him and raise my eyebrows. He reduces his price. I incline my head and step forward again...
An antique dealer invariably winces loudly when someone tries to sell them something at anything less than a very low price.
When you wince and look shocked at a named price, you are sending a signal that the other person that they are breaking social norms. Most people are very fearful of the consequences of such an act and, even in a negotiation, will back down rather than thought of in this way.
When you show shock, it is also a signal that you may well back out of the negotiation (a physical movement backwards emphasizes this). To keep you in the negotiation, the other person will believe they have to act fast, perhaps by making a substantially revised offer (if you are thinking of leaving, a small change may not be enough).