If a listing agent has been told by his client, the owner of the home not to show it until a certain date then they will exactly that. Depending on what state you live in their could be different laws that protect the tenant. If you have questions regarding the legality of this process you should call an attorney. I generally call the selling agent, and advise them that o have a client interested in the property. I then leave them all my contact information so they can reach me when the property is available. If you don't have an agent representing you then you may want to consider getting one.