Guido, I do agree with your discussion relating to the grading of sea shells.

Coin dealers use a system developed over the years. There are several 'recognized' grading organizations, some better than others, who can and will provide 'expert' opinions for a fee. One of them publishes a book that outlines and illustrates the basic basis of each designation in the grades. Looking at thousands of coins over the years does provide the perspective needed for an 'expert' opinion.  

Looking at thousands of shell examples, comparing one to the hundreds and thousands seen in the past, DOES 'qualify' the party for a designation of 'expert'.

The ELEMENTS of that grading system, however, still need to be determined, published and illustrated, as is currently done for coins. Without those notations for the basic basis for each designation in the 'grades' for shells, there really is no  GRADING SYSTEM, just 'opinion'.