Yes, I can imagine how frustrated you are to show your rental to prospective buyers.  Unfortuneately, your current landlord does have the right to sell his property.  In order to sell his rental protperty your landlord would need to allow people to see the unit.  One thing to consider is that the people coming through your house will be your new landlord (at least one of them will eventually be your landlord).  I would suggest that you work with your landlord on this, but, you do have the right to limit the visists.  If your landlord is scheduling people daily, I would ask that they only show your unit once or twice a week.  (that would seem reasonable.) 

Hope that helps!  Good Luck!