Everyone says get the cancellation in writing and "talk" to the broker. I say write the agent and the broker. You want your side of the story in writing. Deliver it in person so you can talk with them.
As Deborah said, if your reason for canceling has to do with the agent/broker, it's another story. Do you feel like you were "pressured" into signing the listing or disagree with the price that you agreed to? This could fall under a 3 day right to rescind for salespersons presenting to you in your home. Now, I've never heard of that for this type of situation but it's a stronger case than you owing $350 on a verbal comment.