Although I have never had a deal "go south" because of cleanliness, experience has shown me buyers and sellers don't always have the same idea of clean.
So, how clean is clean? Is there a standard short of floors you can eat off?
The "standard" I have always heard of has been referred to as "broom clean." Basically contents gone, floors clear and swept.
I am of the opinion that no house will ever be clean enough for any buyer.
In my opinion it all comes down to courtesy. Do unto others...simple. Not always!
Image courtesy of Grant Cochrane at FreeDigitalPhotos.net