you have to rob a bank to pay for it.

Seriously, it's a sequence of cels that is used
more than once in a cut or even in a series. 
Sailor Moon bank cels are especially
desirable because the studio reused the same
sequence showing the sailors' magical
transformations over and over from episode to
episode, so images from these moments were highly
recognizable and at the same time very scarce, in
this case as scarce as OP cels.

CCS bank cels by contrast are not very desirable
because while the studio used the same animation,
they had each transformation drawn and painted
separately to display Sakura's various battle
costumes.  Therefore there are lots of them on the
market, bringing their price down.  

At one point, Madhouse had a set of CCS banks
painted showing Sakura with no clothes on,
intending to have the costume layers painted
separately.  Evidently it didn't work well, so
they had nearly all the banks painted with clothes
on.  But these so-called "Nude Banks" occasionally
show up on the market and bring pretty good prices.

If you slow down and watch action sequences frame
by frame, you can see some small-scale banks, in
which (say) a character running down the street
will repeat the same actions, usually three times.
 This was a cost-saving device to hold down the
numbers of cels to be drawn and painted. 
Hanna-Barbera was notorious for using banks ad
nauseum for this purpose.