you could go to the grand opening, grab a bunch of food and and while eating it, hand out business cards to people and tell them when they buy the part the scanner says is bad and it doesnt fix it - or they buy the cheap rotors that are warped out of the box, or the o2 sensors that dont cycle, to call you to get it done right.
I hate when customers compare our parts prices to the cheap parts stores - all i say is "did you ever hear the saying you get what you pay for" ...even worse - " the parts store scanned it and said i just need an _insert any part here_
It got to the point where i would just put the part on and put the car back outside - whether it fixed it or not - when the customer came back in (after paying the bill) because the problem was still there i would politely ask them if they were ready to pay for a correct diagnosis yet, or if they wanted to take it back to the parts store and have them scan it again, or maybe since it didn't fix it they could get their money back for the free scan. it all depends on how the customer treats me in the beginning during write up. this is exactly why i gave up working on vehicles for the general public.
Does anyone know - if a parts store scans your car and says you need "x".. do they let you return it if it doesn't fix it?