All the seller says is "These came to us in an auction so I suppose the copyright passes along with the slides."
He supposes incorrectly and should a buyer use the slides other than for personal use then they could well find themselves in a bit of bother.
This is just an example of the misunderstanding people have about "art" in general and photography specifically.
Many wrongly believe that owning a physical print, negative, slide etc gives them "all rights" when it does nothing of the sort.
If I had an e-bay account I would mention this to the buyer, but as I don't then I will leave it up to someone else.