it depends on what he is saying. If anything he is saying is factually untrue--for example, he claims there is a termite infestation when there is not, or that it is in flood zone when it isn't--that is potentially both defamation and (because it is lying) tortious interference with economic advantage, or taking wrongful actions to prevent you from making money. You could sue for compensation and/or for a court order that he stop.But if everything he says is either truthful (e.g. say the home has an old roof, and he points that out) or an opinion (e.g. "sure it technically has a water view, but I think it's a pretty poor view, nowhere near as nice as the neighbors have"), he can do that: an person may tell anyone, even another person's possible customer(s) or buyer(s), true things or opinions. You can't legally stop him from talking to people, as long as it's not untruthful what he says. Of course, if comes onto your land, he is trespassing and you could call the police. Even if you are not then there, if you set up cameras and they catch him, you could contact the authorities.