I believe all cels have the sequence numbers
written on them. Hanken don't normally have
anything on them, though. 

Animaxis has such a large selection they probably
don't want to list the sequence number because of
the time involved. Probably another factor
involved is that most of their cels are lower
quality and there are several cels of the same
sequence. They might figure you can just rely
solely on visuals since you have more of a guide
compared to finding only one cel in a sequence on
another dealer's site.

It'd be a little funny to have a cel selling for
several hundred to not have a way to find out its
sequence number. Most smaller dealers will have
more expansive scans so you can see the number, or
you will be able to just email them and ask.