in most cases the seller has already agreed to pay 6% to the agent for the sale of their home - then that selling agent agrees to split it with the agent that brings a buyer. If you come in off the sign you're basically handing the seller's agent another 3% commission - even if you "negotiate" 3% off the top of the house the seller is still paying that 3% to the listing agent.

In the eyes of the buyer it might not matter if you got your 3% off... but that seller probably isn't saving any money on his/her end.