In MA, I believe, there is nothing wrong with marketing someone's property without their permission. If I told a buyer that your house was or could be for sale, and didn't talk to you first, and then brought you an offer for $X, then where is the liability? You can turn down the offer or accept it.
The key here is, the agent marketing without permission has no contract with the owner either! He could spend a lot of time and effort and have someone (such as yourself) sell the property out from under him. It could be a smart way for that agent to get calls though- if the property is in a good location and can get a sign up....you never know what he'll sell because of that location.