Some of the reasons behind a refusal to represent a listing might include...
* Unrealistic pricing expectations
* Unrealistic timing expectations
* Difficult, argumentative sellers (personality clash)
* Poor condition of the property
* Unwillingness to follow suggestions of the Realtor
I'm sure there are plenty more "bullet-points" that the average Realtor could add here. When can we refuse to take a listing? Whenever we perceive that it would be a waste of our time and resources.