Hi Martin,
Speaking for myself, I fill orders in the order I receive them, and I
select the best specimen I have in stock to fill the order I am
filling.  Sounds simple, but of course "best specimen" is often a
largely subjective matter.  Sometimes I just can't decide which
specimen to send because one is more colorful but another has an
interesting pattern, while a third is large and has a nice shape.  In
the end, my final criterion is - which specimen would I most like to
have in my own collection?  If customers make special
demands/requests regarding size, color, or other characteristics, I
try to accomodate them as much as possible, but I also let them know
that the largest (or most brightly colored, or spiniest) specimen I
have may not be the best quality specimen overall.
Paul M.
Northeast Natural History Imports
Rhode Island