Whether this is legal or not usually depends on two things:The laws of the state or country youu2019re in.The rules of the realtors association the agent is a part of.As an example, in California you can market a home which is not yet officially for sale (meaning it hasnu2019t yet been listed on the MLS). But some conditions must be met:You must have a listing agreement signed with the home owner first.The broker must notify the realtors association that there will be a delay before the home is listed (usually in writing with a specific form).The home must still (eventually) be listed.In California, if you donu2019t list a home within a set period (days) after the listing agreement is signed, then youu2019re in violation of the law and you may get fined. Notifying the association that there will be a delay increases the time period before you must enter the home into the MLS, but the home is still expected to be eventually listed (unless itu2019s sold before it ever hits the MLS, which does sometimes happen in these cases).Zillow u201cComing Soonu201dAnother thing that hasnu2019t been said here yet is that marketing a home before itu2019s been listed is actually not that uncommon (though not necessarily with a sign in the yard). In fact Zillow has a u201cComing Soonu201d section thatu2019s specifically intended for this kind of pre-marketing, and some agents use this Zillow feature heavily.All that said, although some of the best agents know how to use pre-marketing like this to drive up demand (and thus the sales price of the home), the question you need to ask is if it really is worth the extra effort (including making sure you know the laws and making sure you comply with them). The agents that do best at this are successful because they have many homes in this stage all the time, having your sign up on just one or two houses may not get you enough business to make it worth the effort.