the problem is you are not selling a business, you are selling a job,
Slacky - you as a sole trader make no profit after you have paid yourself, where as a business makes a profit after all expenses are paid, employees etc and can carry on when the owner is not there
remove yourself and there is no business, its more a job, and why pay tens of thousands for a job when you can get one for free.
you will only get good money for the customer base if you can sell it to someone who already has a business with employees and can take advantage and expand there customer base by adding your customers into there customer base. it would be worth contacting some bigger window cleaning firms that cover your area and offer what you have to them