You present the seller with a written contract setting out the commitments and promises that you and the seller need to agree on and fulfill to make the sale. A well-drawn contract should protect all parties. Once it is accepted by the seller, it may be too late to add or change anything. Everything is negotiable when buying a house. For some reason, most buyers either don't believe it or don't like it.
Negotiation gives the buyer incredible power in making a favorable transaction.
It can also place him or her in a position of immense weakness. Ultimately, how you fare when buying a home is going to be a direct result of your knowledge. The more you know, the better position you'll be in to negotiate.