Hi Jim,
Just a few thoughts re: some of your thoughts:
 
<While a rare shell, let's say a Glory of the Seas cone, will
undoubtably
drop in value as more are discovered, the inverse is true for species
that
are either much harder to find today than they once were>
 
Yes, and how tragic this is, because in most cases the actual demise
of a species is related to the deterioration or outright destruction
of its habitat.  There is no other cause which can have such a
far-reaching negative impact on a species.
 
<I used to own some of the giant "bailer" volutes that I had bought
for $20
in the 1970s. Perfect lips. How much for a really big Melo
umbilicatus
today with a perfect lip?>
 
I don't think this relates so much to the availability of Melo
umbilicatus as to the overall spiraling inflation that the economy
has taken since 1970.  How much did a loaf of bread or a gallon of
gasoline cost then?  It is also related to the fact that suppliers
will charge the highest rate that consumers are willing to pay,
before the point of diminishing returns is reached.
 
<I'd much rather trade with individuals who don't set prices on
their specimens or mine.>
 
They don't?  Would you trade a Strombus taurus for a Strombus
lentiginosus, if you needed the latter for your collection?  Traders
necessarily set values on their specimens, in order to ensure an
equitable trade.  Most don't quibble over exact dollar values, but at
least a general sense of the monetary value has to be considered.
Trading after all is essentially barter, which means selling
something and accepting payment in goods instead of cash.
 
<To predict what any item (a Beanie baby, for example) will sell for
in
twenty or thirty years is darn near impossible.>
 
Yes, no way to know what the next fad will be.  However, the big
difference in collecting a manufactured article like Beanie Babies is
that the number in existence is a known and immutable fact, and there
is no chance that additional "populations" will be discovered.  The
opposite is always true where shells, expecially marine shells, are
concerned.
 
Regards,
Paul M.
Rhode Island