Your post is a perfect example why my garage will no longer fit customers supplied parts. Our insurance company have told us to stop this practise because quite simply in the event of a failure we would not be covered. As a garage we have to be 100% certain where our parts come from, there needs to be invoices seen from a supplier to our supplier and then an invoice from our supplier to us.....in essence there needs to be a paper trail. This can’t happen if a customer has purchased from elsewhere even if they supply an invoice. If a customer was to have a subsequent failure of a component no amount of disclaimers would dissolve us of the responsibility. There have been cases where this has happened and the judge has found in favour of the customer.
Public liability, product liability all go out the window with customers supplied parts and 2nd hand parts.
The quality is becoming quite worrying as it was only a few months ago a chap asked to fit a pair of supplied lamps to his RRS. They would normally cost around £800 each from a dealer.....yours from eBay for only £120 each new, terrible quality made of thin plastic, fixings all in the wrong places and weighed about half of the originals.