Grading by hand is never perfect, so a few units can slip. If more than 10% of the units you received are below the grade the seller declared, you can raise an issue.
Count every unit that landed in a lower grade than it was sold as. In a mixed lot, a unit sold as Grade A that arrives as Grade B counts, and so does a Grade B that arrives as Grade C.
Raise the issue within 72 hours of marking the order received, with photos of the units below grade. The seller then has 48 hours to offer a full refund, offer a partial refund or explain.
Buying part of a lot? You receive a proportional share of each grade, give or take 10%. The same rule applies to the units you received.