The easiest way around it is to to what I do.
Put in your quotes that you do not guarantee any work over anybody else's boarding be it a builder or customer, only your own or lads that work for you.
I don't guarantee any re-skims either.
Did a last minute job for a joiner on Thursday a 25m2 ceiling which I hadn't seen, got there and the boards were not staggered nor were they running opposite to the joists, I explained he reply crack on.
I started putting on and it was flashing on me, skinning up after about 2/3 mins, I got the guy back and asked him how long it had been boarded he replied 18months.
It looked s**t after and will probably crack in about 2 weeks. His problem, not mine