Negotiation is a Funnel
What do I mean by that? I mean that, as the negotiation progresses, there should be a clear line of sight to the end point AND the number of issues to be resolved should be decreasing along the length of the funnel.
The starting point (if you are the seller) is to get a comprehensive list of the changes the buyer is asking for.
Once you’ve got that list, you can pull together your negotiation strategy.
The key is avoiding concessions as much as possible. A concession is a freebie, a giveaway: the more of these you give, the more the other side will (with justification) expect.
That doesn’t mean that one-sided concessions should be avoided totally. Concessions have their place, particularly in the early getting-to-know-you phase of the negotiation. They are a bit like the breadcrumbs you throw in the lake when you are fishing. You should throw enough to attract the fish, but not so much that the fish become satisfied without biting on the hook.
However, once the negotiation proper begins, you should avoid conceding and move onto trading. If, instead of conceding, you trade points, so the other side is giving something too, you are implementing the logic of the funnel.
When both sides have given up all they are willing to give, the negotiation will reach its natural end. You have reached the end of the funnel.