When to Accept an Offer
Crossing a rushing stream is easier if you take the time to locate the stones creating a path across the stream before wading into the water.
The art of negotiating is most required when you are presented with an offer that is acceptable; but you don't know if it is the best you can do!
When it's time to stop negotiating and accept the terms is an art of timing.
While you do not want to needlessly leave anything on the table, you do not want to over negotiate a point and risk losing the whole transaction. Understanding your business model enables you to know when you have acceptable terms. Knowing the other person enables you to know when you have pushed him as far as possible.
How do you get to 'know' the other person in the time span of a negotiation?
You do it by observing how he or she reacts/responds to various aspects of the negotiation. As you discuss terms, make offers, and react to offers made to you, carefully observe the non-verbal reactions of the other person. These reactions become the benchmarks you will need to evaluate how hard you have pressed them when the final offers and counter offers are being made.
No conversation should be treated as idle conversation. If you are not studiously learning something about the other side you should assume they are learning a lot about you. Learn to mask or vary your reactions, especially the non-verbal reactions, to keep them off balance. Negotiating is in no small part bluffing.