as you realise if that part either doesn't work when fitted or if it fails after a while, then I can't give you any guarantees for it
You can say whatever you want at the time.
In 6 months or a year when some part fails and they have no electricity, their freezer full of caviar is defrosting, they can't watch the world cup on TV, etc. they will conveniently forget about that.
Defective circuit breakers can cause fires, as can counterfeit ones. They look the same on the outside.
If someone arrived with replacement brake pads which looked ok, and you fitted them, and a few miles down the road one of them fractured in half, jammed and caused the car to swerve across the motorway into the path of other vehicles, would it be useful to say 'wasn't me, customer supplied the parts, no guarantee' etc.?
Persons that buy their own CU and expect someone to fit it are usually of the type who:
- buy the cheapest thing they could find in B&Q, or even some online tat bazaar.
- buy something which is not suitable such as pre-stuffed with devices, half of which are the wrong ratings.
- don't buy the other essential parts such as glands for where the cables enter, and expect you to provide them for free
- want the job done for virtually nothing
In summary, the kind of customer that no business wants or needs.
If the customer buys the exact proper things and knows why, they are probably capable of installing the thing themselves.