However I am paid to negotiate and buy things for people (not real estate). I have even had people tell me that they will pay me more if I can get them a better deal. I tell them that they don't have to do that. That I charge a fixed fee. [1] (And the fixed fee varies btw.) The truth is I get them a good deal because (as I mentioned in my other comment) this is a game for me that is fun and that is the reason that I do this. The money that I make is secondary (one reason is it's not my primary source of income).
In one case I already had a buyer and a seller agree to a deal. The buyer (my client) agreed to the price that I had negotiated and was very very happy. However when talking to the seller I sensed weakness. So I exploited that and got them a better deal than they had already agreed to. Just for the fun of it and because I could. Just to hear the pleasure in my client's voice. Was worth a great deal to me. Didn't make 1 penny more.
So my point is although I think in general (and a book that I can't remember the name of talked about this concept with regards to realtors) you are right there are almost certainly realtors who act the same way that I do in other words they do act in the client's best interest because it is a game for them like it is for me. I guess the key is finding that type of realtor.
[1] Some people who do what I do charge a fee based on "how much they save someone over their budget". So the game there is that they try to get you to increase your budget while knowing that they will be able to buy for much less so they are playing you knowing you don't know what they know about the market.