How we change what others think, feel, believe and do
Cry foul. Object to how things are being done. Complain about the process.
Take the moral high ground and object to things that may be unfair for other people.
This is particularly useful when you have little to say about what is being offered (how it is offered may be easier to criticize).
An easy time to use this is when the seller is being conspiratorial and pretending to do something slightly illegitimate.
When they apologize, you can ask for something as an atonement from them.
Won't giving me all that disadvantage everyone else? I really think that is quite unfair.
So how is this going to be decided? Are you going to go to other people behind my back again?
Don't you use those high-pressure sales techniques with me!
There are two parts of any negotiation, the substance of what you are trying to achieve and the process by which you get there.
We have a deep need for fairness, and if this seems to be compromised then we will be unhappy. Even if we are getting a good deal, we will worry about others, either that they may criticize us for collusion in the unfair process or that we genuinely care about them and do not wish to see them ill-treated.
Taking moral high ground puts you in a position of superiority and forces the seller to retreat and seek some kind of forgiveness.