I used to run a power tool repair shop.
Cheapo tool brands like Ridgid, Ryobi and the B.S. that Sears pulls with their tools have killed the business.
Good tools like Makita, Milwaukee (TTI owns Ridgid, Ryobi and Milwakuee now), DeWalt, Bosch, etc are worth the trouble to repair.
The cheap crap brands arent.
The good thing is that contractors (the good ones anyway) buy good tools and want them repaired well. You won't make any money off joe-blow homeowner though.
You can't get parts breakdowns or repair parts from Sears for their tools, you have to get them repaired by Sears.
Hilti also won't allow any repairs outside of a Hilti service center or it will void the warranty.
The Ridge Tool Company (Ridgid plumbing tools) is not the same as RIDGID power tools (garbage house-brand stuff from Home Depot)
Anything more involved than a tune-up on a lawnmower isn't worth the trouble these days.
Anything that breaks on a Dremel tool will cost more to repair than replacing the tool.
Air tools are usually worth fixing unless it's a homeowner grade tool.
Here's a list of who is who in the tool business:
Delta, Porter-Cable, DeWalt and Black & Decker are all the same company.
Bosch, Skil, Dremel and Roto-Zip are one company.
Metabo and Walter are one company.
Milwaukee, Ridgid and Ryobi are the same company (TTI) but Ridgid and Ryobi are Home Depot's in-house brands.
Craftsman used to be Emerson or Panasonic but now it's whatever factory in China is the cheapest.
Hitachi is just Hitachi.
Makita is just Makita.
Campbell Hausfeld and Speedaire are the same.
You can make some good money if you can learn to overhaul air nailers. We made good coin on those.
I'd say our biggest money makers outside of the rental business was repairs on roto-hammers, concrete breakers, air compressors and air nailers.
That's all I can think of right now. Hope it helps.
Stay away from lawn and garden stuff, there's very little money in it now that a gas string trimmer costs less than $75.00