I have been renting my current home for approximately three years. A couple months ago I received a letter from the Landlord stating that the property was being placed on the market. It also stated that I may have to show my home to prospective buyers. This was not a problem for me. Although, the other day something happened that just shocked me. I heard some rustling outside my front door. Then, the doorknob began to turn. (Fortunately it was locked) What happened next blew my mind. A key was inserted into the lock and the door was pushed open. To my surprise the realtor walked right into my home. He seemed completely undaunted at the fact that he just entered an occupied home. He went on to explain that he had prospective buyers meeting him there within the next twenty minutes and I needed to leave so that he could show the home. So this leaves me with a few questions.
(1) Is it legal for the Landlord to give a key to the realtor?
(2) Is it legal for the realtor to enter my locked home without even knocking first?
(3) Shouldn't there be some type of reasonable notice that the realtor intends to show my home?
If you have answers to my questions, your help would be greatly appreciated. My Landlord is not exactly a "people person". When I brought my issues up with him he just hollered at me and told me I needed to show the place whenever necessary because he wanted it sold. Please help. Thank you.

