In my experience, the distribution of sales among the various families
of shells depends on whether a dealer's clientele are largely new
collectors or more experienced in the hobby.  Beginning collectors
are more likely to be generalists.  Some will remain generalists
throughout their collecting careers, but of course many advanced
collectors tend to become more specialized over time.  If your
mailing list includes a lot of generalists, especially beginning
ones, you are going to sell a lot of those shells which just have to
be included in every general collection - Epitonium scalare,
Thatcheria mirabilis, Tibia fusus, Stellaria solaris, Scaphella
junonia, Cypraea mappa, Conus textile, etc.  A second factor is that
beginners are often younger folks with very limited budgets.  Some
years back, I used to advertise in Boy's Life magazine, and as a
result I did a lot of business with youngsters who were just starting
out in collecting.  (Interestingly, about 75% of the customers I got
through those ads were girls, even though it was a boys' magazine.)
I have always tried to offer a wide range of species, including the
very commonest ones (who else is crazy enough to list 100 species of
Nassariidae??), so beginning collectors, especially young ones, could
find many things within their budget.  And that, I believe, is a major
factor determining who buys what.  If a dealer does a lot of business
with new collectors, then such species as Architectonica (sundials),
Cancellaria (nutmegs), Trochidae (top shells) amd Naticidae (moon
snails) will be good sellers.  However, more advanced collectors,
statistically, are less likely to purchase such shells.  Beginning
collectors also frequently like to obtain as many new species for
their dollar as possible.  If a youngster gets $20.00 in a birthday
card, she would typically rather exchange it for 10 or 12 sparkling,
colorful olives and cowries than one $20.00 shell.  As a result,
beginners often purchase a lot of low-priced shells like Nassariidae,
Columbellidae, Cerithiidae, Olividae, and Neritidae.  Within the more
popular families, there is a similar trend.  Novices purchase a lot
of cowries and olive shells, fewer cones and murex, and almost no
volutes.  Among the more advanced collectors, I sell more cones and
murex, again because many of them specialize, while the generalists
are also interested in these groups.  I have also noticed that during
on-line shell auctions, the cones and murex seem to induce
particularly competitive bidding.
I did a little comparative study, based on my data base of shells in
my current stock, which I think sheds some light on this.  Of 130
species/subspecies of Olividae, over 85% list for $5.00 or less.  In
Cypraeidae (150 species), 63% fall in that price range.  This I
believe is one attribute that makes themse families attractive to
beginners.  Among the cones (200 species), only 49% make the $5.00
and under bracket.  In Murex (125 species, excluding all the little
rock shells like Drupa, Thais, Acanthina, Morula, etc.), only 20% are
in that price range.  And for the volutes (90 species), a mere 4%.
Hate to close on an ugly note, BUT - several folks have alluded to
"ugly but precious" specimens they possess.  Maybe some others would
care to share about the ugliest shell in your collection, and why you
keep it.  Dealers - the ugliest shell you have ever sold?  Mine was a
specimen of Spondylus calcifer, from west Mexico.  The specimen was
about 9 inches (22 cm) wide, and weighed 12 pounds (5.5 kg), with the
lower valve about a pound heavier than the upper.  The upper valve was
almost 2 inches (5 cm) thick, with an additional layer of encrustation
at least an inch thick.  When closed, it looked like a coral-encrusted
canon ball.  Sticking out of the center of the upper valve was the
4-inch lower valve of a Chama mexicana, equally encrusted.  It made a
handy handle for lifting off the top valve of the Spondylus.
Eventually someone purchased it, and hopefully it still has a good
home on a strong shelf somewhere.
Regards,
Paul M.
Rhode Island