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.