I have been looking at shells on Ebay for several months now, and have even
offered a number of specimens for sale myself.  The shells being offered on
Ebay range from outstanding gem quality specimens to absolute trash.  I
would estimate I have seen at least 30 to 40 misidentified shells offered
(like the Marginella sebastiani George just pointed out, listed as
Marginella goodalli).  But this mistake is at least understandable.  The two
species are both orange colored, white spotted Marginella species, coming
from the same locality.  I have seen some rather bizarre misidentifications
though.  The other day I e-mailed a seller and pointed out that the shell
she had listed as a beautiful Philippine Cypraea was actually Cittarium
pica, a Caribbean top shell.  The same person had a handful of Cypraea
caputserpentis listed as Cypraea eglantina.  In some cases, a rather common
shell has been listed as an uncommon species, such as the Fimbria fimbriata
a few weeks back which was offered as Fimbria soverbii.  One time I opened
up a page listed as a "beautiful shell from Cape Cod", which "would look
great on a chain"  to find a picture of a single, badly beachworn Crepidula
fornicata.
Checking the feedback comments a seller has received is of little value in
my opinion.  A positive feedback simply means that the buyer was fully
satisfied with his/her purchase.  However, just as some sellers don't know
the difference between quality specimens and commercial grade junk, the same
is true of many buyers.  Shells are very popular items, and it is not just
serious collectors who buy them (as shown by the tons of commercial grade
shells sold in bins at tourist shops).  So, if a person offers a box of
acid-dipped cone shells with ground lips as "beautiful specimens", and the
buyer agrees that they are "beautiful specimens", then the buyer will post
highly laudatory comments about the seller.  There is no dishonesty at work
here, either on the buyer's part or the seller's - just mutual ignorance.
This seller is not likely to accumulate many negative comments because
experienced collectors will recognize the shells as junk from viewing the
image, and will not bid on them, while inexperienced collectors, or
non-collectors, will be thrilled with them.  So, the only time a seller is
likely to receive negative feedback is when a serious collector makes the
mistake of bidding on an inferior shell, which is not likely to happen very
often.  And even then, many buyers will not post negative feedback if their
complaint is promptly addressed by the seller.  So, it is definitely a case
of "let the buyer beware".  It takes a certain amount of experience to pick
out a quality specimen, even in a shop, where you can hold the shell in your
hands.  It can be tricky indeed to make such judgements based on a picture,
especially the quality of picture often encountered on Ebay.  If you are in
doubt about the identity or quality of a specimen being offered, perhaps it
would be wise to ask a few other collectors to view the shell and offer an
opinion.  Just as the better mail order dealers eventually become known by
the personal experience of many, and by word of mouth, so too, in time, the
more reliable online dealers will get to be known the same way.
Paul Monfils