Gosh Annemarie, I feel for you. This isn't an easy job. I personally wouldn't list it until it was cleaned up and ready to show. If they have the funds, renting one of the PODS that are available for drop off on your driveway may be an option. It's better for them to have their stuff in a POD outside of the home then have the inside of it full. It takes tough skin, but you're going to have to tell them that no one is going to buy their home if A) They can't see it enough to tell what they are buying and B) An inspector and an appraiser are going to have to come in at some point and they too will need to see what they are inspecting/appraising.
Good luck! This isn't an easy one.