It's a little disturbing to me that we're discussing nuts'n'bolts. I guess the idea isn't insane.
I would argue that anybody trusting their watch to your service would want assurances that their watch wouldn't get trashed. That means you're carrying insurance. More than that, if the watch is going out to unknown owners regularly it's going to need some regular maintenance. You're also not going to get high school kids to clean, like, a $1500 watch; you can do more damage in cleaning it than you could leaving it alone if you don't know what you're doing so at a minimum, you're discussing semi-skilled labor.
But yeah. Might be viable.