The problem is that the seller's must pay for both the buyer's agent as well as the seller's agent. And in order to get into MLS, you have to hire an agent to represent you. It's a horribly broken system where agents are essentially a cartel you have to do business with in order to sell your house. Everything has changed with MLS listings being online and so many real estate transactions have become boilerplate, with no major variables except price negotiation, which really isn't that difficult either.
If you're not familiar with the MLS system, it's something that is controlled by the Realtor association it's essentially a network of databases of every home for sale or rent that is represented by an agent. You have to have an agent in order to be listed that system and practically every real-estate website, including Zillow and Trulia, get the vast majority of their listing from the MLS systems.
Some smart agents realized how utterly ridiculous the role of the buyer agent has become in many situations (If you know the 80/20 rule, this is sort of the 99/1 rule: 99% of the value of a buyer agent comes from getting into MLS and it takes about 1% effort to do that). In response, they offer their services as "flat fee MLS" and they charge people a couple hundred bucks to play ball in the MLS system and you get to represent yourself in the actual sale process. You're paying $600 + 3% buyer commission, instead of a straight 5-6% total commission.
On a $300,000 house that will save the sellers $8,400.