Well, looks like i get first crack at the grading  question!!  (and
even i am glad to see the end of the hurricane season, even though i
started it..).  There are several varieties of grading-, including: 1)
the "ideal" system, which i'msure someone will learnedly deposit in you
inbox soon enough, but which virtually nobody actually uses,  no matter
what they claim, 2) the "dealer's only" version, which allows a few
"bold" dealers to list 85% of their offerings as either GEM or F++++
(not to be confused with 2a) the "wishful thinking" system, which, while
similar in results, has different motivations), and 3) the system used
by most dealer, and by default, collectors.  It is somewhat more
generous than  the "ideal", yet realistic and less starry-eyed than
versions 2 a 2a, above.  It centres around the "F+" grade, as in"just
how wide can i make this grade, anyway?", since for most species, a
grade of "F" is like the kiss of death  on a dealer's list. (ideally, an
F+ specimen could be confused with a  GEM shell by a casual observer not
looking  too closely.  We do NOT live in an ideal world, however!!).  in
a nutshell, here it is (for what it is worth.. the same specimen can,
and often does, end up graded 5 different ways by 5 different dealers-
in practice, one must find out by actual experience what various grades
mean to the dealers you purchase from, and how consistant each is in
their application):
        GEM: A  shell visually PERFECT under the magnification normally used to
view it (ie, 0 power  for normal-sized, and 10 power for micros) .  No
matter what a few dealers' lists look like, this is rather  rare, except
in some spp of  cowries and Marginellidae.
        Gem- or F++ (note: in my opinion, grades like F+++ are absurd):  ALMOST
perfect- one or  two extremely tiny flaws or mends.  A "superior" shell.
        F+:  A good specimen- worthy of exhibiting  in most collections.  A
couple of mends or other types of "flaws", but nothing  that
"signifigantly" damages it's natural aesthetic qualities.  As i said
above, this tends to be the broadest category, since shells listed as
less than F+ tend to sell slowly, and at rather low prices!!
        F (Fine):  Not a dissaster, but signifigantly flawed in some aspect.
(howver, many specimens can be "F" on one side, yet far better when
flipped over, so for a display, two such specimens, oppositely impared,
would look the same as two completely great specimens, and cost a
fraction of the price!!)
        F-:  Not a great shell, by any definition.
        Good:  Not good.
        Fair:  Recognizable 8 times out of 10 as a particular species.
        Poor: Recognizable (usually)  as a shell.