I once bought a cel from a guy in France (well,
long fraud-riddled story, but he sent me *one* of
the two cels I paid for... and after I started
filing fraud charges he finally refunded the 
money for the other cel :P) and he shoved the
cel into a padded envelope... unfortunately, he
didn't pay attention as to HOW he shoved it in
there, and when he sealed the envelope the cel
was bent and ended up with a permanent
crease.  It was very painful to look at for
me...=(  (Especially since he took forever to
mail the blasted thing in the first place!)

Anime-cel.com also used to use flimsy cardboard
(love those guys, nice cels, but the post office
has a field day with anything that can't 
withstand an elephant force against it) and 
the package often came looking like it went
through a paper shredder... I had a few cels from
them that ended up a little curled or slightly
(but not permanently) creased due to this...
luckily no real "horror stories"...

I really like Anime Shop Vi's packing... it would
take a natural disaster to bend that plywood
and damage the cel.  =)  That was probably the
best I've seen... although Taro, Animanga, 
Nichibei and Usagi's House all pack well, too
(although the last two took me several agonizing
minutes trying to figure out how to get the cel
*OUT* of the package)...

Lady Z