First of all the last guy may have fixed the problem at hand. Did you ask your customer what was done ? Have you never repaired a unit only to be called back for something else that went wrong ? I would bet that all Craigslist ad guys are not hacks. Could be a journeymen doing side jobs. Do you work for yourself ? If so, believe me you will gain more customers by saving them money verses changing equipment. Yes, replacing equipment is sometimes the better choice but with the little details you gave here
P.S. I know it sounds like everyone is beating you up but don't think we don't like your attitude to DO THINGS RIGHT !