Cels Forum

Recently I got very interested in buying cels.  Curiously 
I found the price of cels usually differ in a big degree.
Say, the same is Marmalade Boy, the same is Yuu, the same
size of the image, the same has background, but the price of 
one is $30, and another could be $130.  I feel really confused
by this price range.  How do they determine how much to be the
price to set up?

Also although I buy cels because I like the anime, I do want
it to be a valuable thing that wouldn't fade too fast with
time.  What would be the good rules/standards to choose a 
valuable cel?

1)Better the character image to be large, facing front, with 
eyes open, centered on cel, looking nice, better if it's a 
smiling one?
2)Better if the whole image fills the cel and when there's no
cut-off body parts?
2)Needs to have background?
3)Better to be a main character?
4)Better to have more characters on the cel?
5)Better if the colors are bright (instead of shadows that
made the whole cel image grayish)?

Or
1)It matters more if the cel looks good as a whole.

2)Depends on how rare this particular image would be similarly
repeated.

3)Depends on how many cels there are in total in the production
of this series.

4)It matters more if this shot signifies a special episodic
moment.

5)It matters more whether you think it's nice or not, and to
anybody the same.

I am really confused on choosing which kind of cels to be worth
the price.  If you got an opinion on those questions, please 
help me out, thanx!!