This job posting struck me, because there are guys who are going after this business of washing windows.  To make a business of it I would think that you have to offer washing windows as one in a series of services, like installation, replacement, and so forth--you know, buying windows.  The job info is here:

1-2 years professional experience.
Most of your own tools (bucket, wands, blades, t bars etc.)
Max weight 225 lbs.
Pass drug & background check
Great attitude
Desire to make good wages
Contract or piece work for well qualified pros
Reliable transportation

805-896-6989, Damion.  His email: 

• hntdz-4896088602@gigs.craigslist.org