While many collectors, especially the more advanced ones, consider a
shell with a chip or a bit of repair as something to be avoided, it
is certainly true that many other collectors view such specimens as a
golden opportunity to acquire an uncommon or rare shell at a low
price.  More than once I have had people call or write me asking if I
might have a slightly defective specimen of a certain shell that I
could offer them at a reduced price.  One fellow called and asked
outright what I wanted for my "worst" golden cowrie.  Personally, I
view such requests as an opportunity to unload a specimen I would
otherwise never have sold, and to at least regain what I paid for it.
 Even if I sell it for less than I paid, it still benefits me more
than it would sitting on a shelf gathering dust.
On a related note - it is not only rare shells that are subject to
artificial repairs.  The commonest shells generally will not have
repairs.  Who is going to bother repairing a snakehead cowrie or a
bat volute?  However, once you get into shells that retail for $5.00
and up, you have to be careful.  Dealers often purchase such shells
in quantity lots.  The exporters they buy from purchase their stock
from many sources, and have no control over how those front-line
suppliers handle their shells.  And, when the exporter subsequently
makes a shipment, let's face it, a lot of 50 map cowries isn't going
to be checked as carefully as a single Cypraea leucodon would be
checked.  Consequently, in most shipments I receive, there are
specimens that I have to reject, either because of severe natural
flaws, or because of human tampering.  Fortunately, repairs to shells
in this price range are usually easier to spot, because they simply
are not as well done.  Those who produce fake Cypraea sakurai are
true artists, albeit dishonest ones.  Those who grind down the lips
of cone shells to remove chips are not.  Some of the commonest
"alterations" to watch for are:
FILED LIPS - Everybody knows this one, at least as it pertains to
cone shells.  It is also a frequent problem with volutes; tun shells
(Tonnidae); moon snails (Naticidae); turrids (Turridae); and other
thin-lipped forms.  A Conus lip that is filed flat is easy to notice,
but one that has been filed and then resharpened requires a closer
look.  I once gave myself a nasty cut by running my thumb along the
lip of a newly-received Conus generalis (a common method of feeling
for tiny chips).  Someone had sharpened the thing to the point where
you could have shaved with it (well, not quite, but close!).
SHARPENED APEX - On slender, tall-spired shells, the apex (the
pointed end of the spire) is easily broken off.  Some suppliers
resharpen them by grinding, with or without subsequent polishing.
Augers (Terebra) and miters are the usual victims of this, but I have
also seen it on Tibia.  Such species usually have a specific normal
"spire angle" which extends all the way to the apex.  If the spire
angle suddenly curves inward near the apex, be suspicious.  This is
usually easy to spot, as it destroys the sculpture and color pattern
of the early whorls, as well as changing the spire angle.
SHARPENED SPINES - As one might expect, the delicate spines of some
Murex, Spondylus (thorny oysters), etc. are easily broken, and some
suppliers will resharpen them.  This is very common, especially on
thin-spined murexes like M. pecten, M. nigrispinosus, and M.
troscheli.  If the shell has a row of spines of gradually reduced
size, and one spine in the row is abruptly shorter, check it.  Again,
such spines have a normal angle of reduction, and a filed spine will
have either a wider angle, or an abrupt change in angle along its
length.  Also, it is often more sharply pointed that the natural
spines.  This is fairly obvious once you have a little experience
with it, but can easily fool someone who doesn't know what to look
for.  Spondylus sometimes have resharpened or reshaped spines,
especially species with long slender spines like S. imperialis.
Lambis often do too; and I have found sharpened spines occasionally
on Aporrhais and Latiaxis.
FILLED HOLES - The usual sources of holes in shells are boring
organisms and predacious mollusks.  The former can produce winding
tunnels at various angles to the shell surface.  The latter produce
round holes, usually quite perpendicular to the shell surface.
Filling them is easy - just press some sort of putty or other
material into the hole and wipe off the excess.  Finding such repairs
is not always so easy, especially if the color of the fill material
resembles the shell color, or the shell has an intricate multicolored
pattern.  More than once I have placed a shell in the ultrasonic
cleaner to remove a bit of grunge, and afterwards found one or more
holes in the shell.  Ultrasonic cleaners do not drill holes in
shells, so apparently the holes had been filled with some material
that the UC removed.
XENOPHORA ATTACHMENTS - Personally, I feel that replacing an
attachment that has fallen off is acceptable, if it is neatly
reattached in precisely its original location and position.  However,
I have received some Xenophora (carrier shells) which had additional
objects glued onto them, to make them look more interesting.  This of
course, is not acceptable.  Such artificial attachments are not
usually too difficult to find if you look closely.  Some of the glue
is usually visible; and, once you get a feel for it, such objects are
often fairly obvious by their angle or mode of attachment.  Depending
on what kind of glue was used, the additional attachments may fall
off if the shell is placed in bleach; and they almost always fall off
in the UC.
Any collector concerned about "shell doctoring" has to have some sort
of magnification device.  A stereomicroscope, of course is best (there
was an ongoing discussion of such instruments on Conch-L some months
back).  This will reveal even the fine scratches left by sandpaper on
a shell lip.  But at least a good quality hand lens or "reading glass"
is essential.  It's often surprising what you find when you view your
shells larger than life.  Even a 10x magnification can reveal many
natural and unnatural features you might otherwise have overlooked.
Paul M.
Rhode Island