That is a tricky one Guy!. Do you know, or know someone who knows the window cleaner in question?, if someone can vouch for the guy, that he not going to try and mess you about, that should put your mind at ease. Otherwise, I really don't know if there is a 'legitimate' way of doing this, after all a window cleaners customer is just an address. You will have to go on your gut feeling about the guy!. I would suggest you get him to introduce you to the customers in question, that way you know that they are aware of what is happening. Once you get in there, just make sure you do a good job, and be nice to them, in most cases that will be enough for them to stick with you. I got started window cleaning by buying part of an existing round, but I was introduced to the guy selling by my mates brother in law, also a window cleaner, he knew the guy selling, and could vouch for him. Some papers were made up on the computer and signed by all 3 of us, but I doubt they would have held much water from a proper 'legal' point of view!. Be aware though, that customers can be fickle, and may just decide, for no apparent reason, that they don't want a 'new face' cleaning their windows!.