Is it legal for the landlord to sell & show property that is under lease. What is the law?

When you rent a property you pay for the rights of exclusive possession and quiet enjoyment.  Thus, unless your lease gives your Landlord the right to enter your property he has no right to do so.  You may refuse entrance and call the police based on trespass.  But, most leases give the Landlord some rights of entrance, you will have to read your lease carefully so determine exactly what your rights are in this situation.    Good Luck!