The obvious answer is that they want to give the previous name to their next boat.
Occasionally I've seen for sale ads in USA where it states that the name is reserved. But I 'm not sure that would be enforceable. Its obviously better to change the name before the boat is listed for sale.
Now, it could also be that the boat, under its old name, has some history that they would prefer people not to know. If interested in the boat then a bit of questioning, preferably face-to-face so body language/reactions can be assessed, would be warranted.