How we change what others think, feel, believe and do
Make one or more items that you need or want to be things on which you will not concede at all.
When the other person tries to bargain with you on a non-negotiable, state that you are not prepared to negotiate on this thing. If they persist, just use a broken record response.
You can distract them from any persistence by offering a concession on something else.
I'm sorry, I cannot include the carpets. They were my parents.
I must have four wheel drive. That's a nice car, but I must have four wheel drive.
Sorry, son, homework comes first. It may be your best friend's party, but you can't go until homework is done -- to my satisfaction, too.
When the other person believes that you are not going to concede on a particular item, then they have the choice of terminating the negotiation or giving in on that point.
Unless they have a walk-away alternative, then the thought of terminating relationship will not be a good option for them.