I think what seanie is saying is if something happens a pulley the motorfactors the mechanic gets his parts off would stand over it as they know them from buying parts.
I get it here in the bike shop every day, lads buying parts on chain reaction for cost then bringing it in for fitting 90% of the time the parts are wrong size or spec for there bike.
So I just don't bother with it anymore its a pain in the balls.
I'm sure most mechanic be the same about people bringing parts they got off the net or cheap somewhere that are not up to there spec, and if it s.hits it's self a week down the road he was the last at it..
That's my 2 cents on it maybe I'm wrong.
I'm like Joe I give over the car and I let my mechanic do it from top to bottom.