Sounds like a brilliant idea to me... customer surfs 'tinternet for fault (no need to go near car at all), buys part from 'tinternet (junk made in China), brings it to us to fit. Having paid us to fit it which does not cure the problem we can then make an additional charge to diagnose and cure the real fault. Using this method the customer can give us more money than they would have done if they came straight to us. What can possibly go wrong?
Example: Young customer (at "Uni" don'tcha know) with a Clio II, immobiliser problem (got to be, it's all over the 'net). I had trouble convincing him it was a corroded main feed terminal on the starter motor. Also had a battery drain or faulty brand new battery as it went flat very quickly after fitting, perhaps the missing alternator belt had a minor effect on this?
Joys of the motor trade, Shaun