That is a popular misconception. Not to take away from the negotiating skills of anyone, but to be an effective negotiator in any field, a person must have full knowledge of the situation. In this case, having a knowledge of real estate pricing, customary fees, real estate contracts, comparable options and the overall market is VITAL to negotiating.
Also, PRACTICE MAKES PERFECT! For someone to negotiate 1, 2, or 3 real estate transactions in their lifetime and think they are doing themselves a favor is questionable. Of course, working with an agent that only sells a 4 or 5 deals a year isn't really all that helpful either. Find an experienced full time agent and you will be much better off.
Lots of times, in by owner transactions and negotiating with home builders agents, the general public feels they may be saving on paying 3% to an agent and they end up giving away 15% because they aren't knowledgeble about the factors above and don't have experience on thier side.