What are you willing to sell your unit for? Know this and you can be confident during the negotiations. When you hesitate, change your mind or appear uncertain, negotiations will be far more difficult.

The quickest way to lose control of a negotiation is to become too emotionally involved. Obviously, buying and selling something as expensive as a house is an emotionally loaded experience, but cultivating a detached manner will help convince your opponent that you are a competent negotiator who is comfortable with his/her position. Being too emotional will always be interpreted as desperation and that gives the upper hand to the other side.

Get to know the other persons background and history. What is their motivation? Knowing their motivations will give you extra information on why this deal may be extra important to them. Such as:

Has a seller already sold their home?

Has the buyer been looking for a long time and finally found their dream home?

Is the seller or buyer relocating?

Additionally make sure that you understand the other persons position. Deals fall through all the time because of a misunderstanding. If you are not sure what the other side is saying then ask for clarification.

If you are very far apart you might want to politely tell them you enjoyed meeting them and appreciate their interest, but that you're so far apart that further negotiations would be fruitless. If they are serious they will likely come back to the table quickly.

If you have negotiated to your target sale price be careful not to jepordize the deal by getting greedy. Get agreement and be satisfied that you reached your goal.