All is well with what you and hundreds of others do provided you pay attention to 1 cardinal rule.
Do the job properly as if it was your mother's mower.
There are way too many ratbags who pick up mowers from the rubbish , bodge them up so they almost work, tart them up so they look pretty, then flog them off to the unsuspecting public.
Too many times a unit with a failing ignition module gets tossed out because the module is an expensive part to replace.
Fred the fixer finds it, pops a new spark plug in it finds it runs great the gives it a polish puts a high price tag on it and floggs it off.
Joe public buys said same "restored" mower, it works for a month then fails. He takes it to a shop who tell him it will cost 3 times what he paid for it to repair it ( because they want to sell him a new mower )
They tell Joe Public that all the back yard mower men are sharks & thieves, just one lower than a scrap yard owner and all second hand mowers are trash.
This then makes life very hard for honest repairers who do the job properly.
We get way too much Amercian politics down here but one thing I did notice is every candidate made disparaging remarks about used mowers and used mower sales men.
Even worse was the clapping and cheering that followed which says a lot about the general attitude to used mowers and those who repair them.
The tragic bit is from what I have seen, it is pretty well deserved.