5) I think there are a number of factors in how popular a 
given cel is:
- the popularity of a show
- the poplularity of the particular character (different 
characters from the same show can be VERY different in 
popularity)
- the quality of the art for a particular show.  Some shows,
such as Shamanic Princess, which is an OAV/OVA (only 
released on video - not a movie or tv show) have much better
art than some tv shows, such as Sailor Moon, and so the cels
may be more "popular" or more expensive (popularity and 
expense are very related!)
- the characteristics (personality and appearance) of the 
character.  Some characters are popular because of their 
personality (Tasuki or Nuriko from Fushigi Yuugi come to
mind), while others are popular because of how they look 
(like Larva from Vampire Princess Miyu, or Deedlit from 
Record of Lodoss War)
- the pose of the character.  Some people like action shots,
some like close-ups, others prefer to see a character
acting "typically" (a wizard casting a spell, a fighter 
fighting, a crybaby crying, etc), still others prefer to 
see a character in a particular outfit.
- the size of the character.  Bigger is better.
- the scene.  Certain scenes are BIG sellers.  Many people
have wishlists of cels they want, and its often based on a 
particular scene (Nuriko from Fushigi Yuugi comes to mind -
I don't want to give anything away, and FY fans will know 
which Nuriko scene I mean)
- opening, closing and bank cels (cels that are used 
repeatedly because the scene is seen often, such as an 
attack or transformation sequence from Sailor Moon).  In 
most cases, the opening and closing scenes are better drawn,
the only exception I can think of is Shamanic Princess.
- does the cel come with a sketch or background, and are 
they stuck?  One big exception to this, to my mind, is 
Sailor Moon cels, since the backgrounds are never matching
(the background sold with a cel is not the background used
with that cel in the show).
- cel sequence number...If it is an ending cel, a start cel 
or another key cel.  A key cel is a cel drawn by a key
animator, and therefore better quality.

whew!!  That's it for that question!! 
Az