I don't think flea market sharpening would work. People wouldn't expect a sharpening agent and wouldn't have item to be sharpened with them. Also, can you sharpen while they wait, or would you need time to get all of the items done if there is a demand?
I think the money in sharpening today would be for saw blades for contractors and craftsmen. Some people sharpen, but not worth a hoot. To get repeat business you would have to do an EXCELLENT job producing factory like sharpness.
As an example, I purchased a high dollar anti-vibration table saw blade a few years ago.
It leaves the cut smoother than a jointer would. When it is sharpened, if one tooth is not sharpened properly, it will destroy the usability of this blade for sawing and joining without further steps. The tip of every third tooth is rounded. Can you even sharpen the blade?