Lets say there was an automatic transmission slip.
I would tell the customer a proper job to repair (guarantee it will work as intended when it leaves) it would cost say $3500.
I could do the same repair with the pile of spare clutches I have in my garage from past builds for $0 in parts and my labor cost. I didn't replace any seals, bearings, bushings, clean the valve bores, replace the TC, check and fix tolerances. Doesn't matter if it breaks down again in a week, because 1. I know I can fix it again, 2. its probably going to cost me $0.00 to do so.
If youre car was only worth $3500 in running condition, then well it's literally not worth the proper repair. I can't sell the half ass repair because I can't guarantee you won't be back here just as soon as your tires hit the road.