We explain to them ahead of time that it is rare but something could go wrong on an installation which would damage a piece of material. If they buy it from us, our installer walks inside and we cut a new piece. If they do not buy it from us, tough luck. We will install it of course but no warranty at all on the materials, and while 99% of the time there will be no problems at all there are still no absolute guarantees.
This is exactly how I do it for garments or embroidery files that are brought to me from customers. Same thing. If they buy all the inputs from me and something happens, they never know about it. All they see is the wonderful finished product. If they supply garments, files, whatever, they have to assume the risks, because in most instances, I have no idea where their inputs came from. Not the same in terms what goods are being talked about, but how to handle those situations are the same.
I would also suggest having a waiver signed. Not just have something on the counter talking about it, but something that they actually put their signature on, otherwise, it then becomes (s)he said - (s)he said and they can always claim that they didn't "see" the sign that mentioned the risks on the counter.
No warranties of any kind on product supplied by customer.
