tiger
I used to have my own little decluttering business and still do it for friends when needed. It is very rewarding when it all goes right and I loved doing it.
My main customer base was businesses, everything from solicitors, architects and art galleries. All of my recommendations were word of mouth and I never advertised.
It is very hard work and I offered a complete service that included the decluttering, bagging, labelling (archiving if neccessary) and despatching goods to tip/charity shop/auction.
If you do decide to go into it then be prepared for the clients who want/need/book your services and are still unable to part with a single thing (very frustrating) or negotiate over every single item. I have had clients trying to make 'deals' with me along the lines of "Okay I shall throw away the broken teapot but I get to keep the fourteen pepperpots". 
On the other hand I have had clients that embrace the procedure whole heartedly and worked side by side with me. They are the most satisfying and when I have left at the end of the day I could almost hear the whole house purring.
Also please be aware that some extreme hoarders do have mental health issues that should be dealt with by a mental health professional.
Good luck if you do decide to go for it and PM me if you would like any advice.
[hgrin]