So, one has to choose between being a shell collector or a "shell purchaser"??
By that criterion, I guess there is no such thing as a coin collector, doll
collector or paperweight collector.  These folks typically purchase most of
the items that go into what they call their "collections", and they do so for
the same reasons that shell collectors do - they lack the leisure time and the
financial resources to travel the world obtaining objects in their countries
of origin; plus the fact that traveling to the country of origin doesn't
guarantee you will find what you are seeking.  Therefore, they rely on
importers, middlemen who purchase from collectors at the source.  If it were
not for such middlemen, or "dealers", building a comprehensive collection of
just about anything would be only a dream for anyone but the wealthiest.  In
fact, financing a personal shell collecting trip to a foreign country might
well be viewed as a more indirect and extravagant method of purchasing shells.
As for collecting in quantity, a dealer is a distributor, and doesn't purchase
more of a particular species than there is demand for by collectors.  If a
dealer buys 100 specimens from a local collector, and then resells them two at
a time to 50 different collectors, the impact on the molluscan population is
the same as if those 50 collectors all traveled to the site and took home 2
specimens apiece, which apparently no-one would object to.  The difference is,
using a middleman makes it possible for those 50 people to enjoy the hobby,
while otherwise perhaps only 1 or 2 of them could do so.  Also, while 100
specimens might sound like a lot to someone without much personal experience,
consider that many species are collected by the MILLIONS annually, for use as
food, yet the populations continue to thrive.  In my home state, each resident
is allowed by law to take 1 bushel of scallops (Pecten irradians) per day,
without purchasing a commercial license (obviously those with a commercial
license can take far more than a bushel a day).  The shellcraft industry which
makes all those trinkets you see in tourist shops purchases such species as
Cypraea moneta, Cypraea caputserpentis and Cypraea tigris by the ton
(literally!  Commercial exporters offer them at a per/ton price).  And yet
there is no shortage of these species - for such species as these, 100 or even
1000 specimens is an insignificant quantity.  But - what if a dealer purchases
100 specimens of a really rare species?  Ordinarily, that doesn't happen,
first of all because rare shells are not available in quantity - that's what
rare means - and secondly because there is not sufficient demand for a shell
that costs several hundred dollars.  If a dealer got 100 Cypraea valentia, it
might take him 10 years to sell half of them; and, if such a species became
available by the hundreds, it would no longer be considered rare, and would no
longer sell for such a high price.  In the final analysis, the quantity of any
species, common or rare, that is collected by or for shell collectors is
necessarily trivial compared to the number of living specimens that exist; and
is also trivial compared to the numbers of specimens that are harvested for
other purposes.
Paul M.