Even when I was subbing 30+ books per convention I attended, I was never 100% on what books would be 9.8s. I had a hot streak in 2012 of 50 or so in a row. But then I had 3 Walking Deads miss in the same submission. If you are a very good grader to CGC standards, you can confidently say " I believe this is a strong 9.8 candidate," but that is about it. I actually had the book that I thought was my best 9.9 candidate come back a 9.6 LOL.
So for all n00bs, if you are paying inflated prices for raw books based upon what the owner predicts they will grade, either get a money back guarantee, or have them grade them before you buy them. Otherwise, study and become a good grader yourself.