I think it could fly if there is a market in your area for used fixtures. To rebuild them in volume is a better idea than one-off, but you would need to figure the total labor/costs involved, namely:
-Removing the bad fixture;
-Determining the problem and what needs to be replaced;
-Time to disassemble, clean and re-assemble the fixture;
-Allow for testing and burn-in to make sure it works;
-Warehousing the repaired fixtures until someone buys them.
Or perhaps if you have customers with a large number of fixtures, work out a repair/exchange program where you charge a calculated rate for fixtures where the old one is exchanged for a rebuilt or a second price for just rebuilt fixtures without exchange.
So yeah, it could turn into a nice sideline if the market is there.