I fully understand wanting to do the right thing.. But there is SO MUCH that can go wrong that YOU have no control over.. From pesticides to swarming because he did not watch them close enough being so far away..
So long as you sell a good healthy nuc you are confident in, the queen is laying well etc, there is no further liability on your part...
Once it rolls off of your property, they could damage the queen in transit or pulling the frames, etc, etc, etc, etc...
IF.. he is a good customer, or a good person etc, I may go out of my way to see that the nuc is "fixed up" or replaced, if they/he is a pain or is nasty, has an attitude etc then they are on their own. I will work with an idiot if they are trying, I won't go out of my way for someone who does NOT try.