In this instance, you need to have a clear understanding of what the terms are.
Many people offer guaranteed 9.8 pre-sale signed books. If you don't get a 9.8 then you haven't gotten what you paid for. You are entitled to a refund, at very least.
Others will say "I have 10 NM/NM+ books" and if you order one, you get whatever they come back. That is dicier. What if some come back 8.5 or 9.0? That is nowhere near NM/NM+
But under no circumstances on that pre-sale are you entitled to a 9.8.
If you want a guaranteed grade, negotiate for a guaranteed grade. Otherwise, you get what you get.
That's how I see it. You get what you get. And on the SS, what if the person who is signing pen explodes?
A good facilitator will bring excess copies to account for unforeseen variables. Assuming, that is, that the facilitator is providing his own books and not accepting your particular books. If you are sending in books the facilitator should not be guaranteeing 9.8s.