As these deeds are titled to a specific week and unit, it would be correct if it was deeded as a non-lock off. If I was in the position of selling it, I would state it as the deed declares it to be, then a buyer could not go back saying that you had promised them a lock off when technically it is not, even though you have the ability to. I think it is important to accurately represent what is being sold and I see nothing wrong with what the seller has done in this listing. It is what it is.
Knowing that it can be locked off is a separate issue and unless you really know the resort and the policies of Marriott, you might not know that it can be locked off. As a mass seller, I'm not sure it is fair to expect them to know the ins and outs of every timeshare system. As long as they are representing what they are selling accurately, I think that is the extent of their "duty of care". How we use it as the end user is not really of concern to the seller nor should it be, that is up to the buyer to know how to use it properly.