I would seek out about a dozen clients who cared about carbon and hopefully had small lawns, rich aging hippie types. I would charge about twice what normal lawn service costs; the whole group would arrive and cut lawn and do any other work en masse, otherwise I think it would take too long.
The overall goal was to have enough work to have 1 full day's hard work each week, for which each person would earn about $200. I wanted to design the business explicitly to provide a way to pay the cheapest rent in my area (half the cheapest two bedroom apartment) and a little left over for food and etc.
I wish someone would try this. However, the person who actually starts it for the first time will probably find themselves working full time for quite a while, and it won't be a part time backup job for them.
A side note on lawn mowers -- they are almost all not work horses or indistructable, particularly the $50 ones you will get used. They are garbage. Briggs and Stratton has a good reputation, but I am not sure they are currently living up to it. A good rule of thumb is to get the best ratio of engine displacement to cutting swath you can get. If you get the used ones, get three and expect to be constantly repairing them, which will be fun for a while and then get old.
I like the carbon free lawn service idea. My current mower is actually an American lawn reel mower. Coincidentally, I bought it on craigslist for $35. It's from 72, and still cuts like a champ as long as your willing to maintain it. The "carbon free" service would work great in my area where there is a high density of people in town homes with small front lawns.
Maybe YMMV on the small lawn mowers. But I have had extremely good luck with them. I think that most of the problem people have with mowers is failing to maintain them properly. In my experience, if you can buy a used mower that will start quickly without much hassle it can generally be kept in decent working order for a long time. But purchasing one of these used mowers was why I said the idea scales. It's pretty easy to get enough work out of a used mower to buy a better quality mower.
Thanks for sharing your insight. I hope someone jumps on the reel mower service.