As levelheaded as you think you are, when you're fighting with a seller over adding a hose to the dishwasher because the water drains on the floor, it's easy to lose your cool. Having an agent to write the requests objectively and forward them to the seller saves you the trouble of getting overly emotional about the deal.
Say the seller won't budge on the dishwasher hose and you want to adjust your offer. The agent can handle that part calmly, too. Experts advise that you let the agent take the heat in difficult negotiations.
The best way to make a deal is to look for the positive part of every offer and counteroffer, and never let the other party see you make a sour face. Often, the most effective way to do that is to present the face that's doing business -- not your own.