As said in the other posts, Western cel collecting and anime 
cel collecting are two *completely* different things.  My 
information will be about anime cels, because the Western 
market is far, far different.  Also, I'm going to stick to 
*production* cels in this post, see the links at "FAQ/Resources" 
for more info about different types of cels.  I've found that most 
cel collectors seem to begin where he or she sees a cel for sale 
at an anime store or a website, purchases it out of curiosity.  
Then, that person is drawn into the world of cel collecting 
like a moth to a flame.  Cel collectors are a relatively small 
group.  I do not believe it will ever beocme a "mainstream" 
hobby, although the number of anime cel collectors have 
definitely increased, especially due to the ability to purchase 
cels on the Internet.  The majority of  anime cel collectors 
purchase their cels off the Internet.  eBay is a tricky thing 
when it comes to cels.  There are many "newbies" to cel 
collecting on eBay, people who pay ridiculous sums of money
 for a cel because they do not know what it is worth (or don't care).  
A superb example of this is the current popularity of the 
television show "Gundam Wing" which has recently begun 
airing in North America.  Before it aired in North America, 
Gundam Wing was an average anime series, with cels that 
were relatively easy to find, and were in a moderate price 
range (you could get a good one for under $100).  However, 
after the show began airing in North America, the price of 
Gundam Wing goods on eBay (and in retail stores) skyrocketed.  
Cels of a favored character, "Duo", more than quadrupled in 
price, and other Gundam Wing cels too.  Gundam Wing fanaticism 
is pretty crazy, and dealers are definitely aware of this.  
Anyone can get into cel collecting; there are $10 and under 
cels, and also more expensive cels.
Popularity of Certain Cels-
1.  How popular the series is.  Supply and demand.  If the 
show is very popular, prices are obviously going to be higher.
2.  How many cels the studio released (or if they released them 
at all)-
.  The animation studio usually releases the cels to the public 
as part of the way that it makes its profit.  However, sometimes 
the studio doesn't release the cels.  This occured with the studio 
GAINAX, with the series Neon Genesis Evangelion and Kareshi 
Kanojo no Jijou.  Thus, the only way to get cels from this series 
is animators either stealing them or having cels given to animators 
and other people affiliated with the studio as gifts.
3.  Length of the animation-
Cels from a one minute trailer are going to be more expensive 
and uncommon than a cel from a six year tv series.
4.  Rarity of the character/scene-
One example of this is the character Black Lady from the 
children's series Sailor Moon (also airing in North America)  
She only appears in two episodes, but is still an important character.
5.  Quality of the image-
Depending on your preferences, a full body shot or a close 
up of the face is going to be more expensive than a one inch 
cel of a tiny character.  Also, matching backgrounds, a non 
stuck sketch included, etc.

This post is too long and the forum won't accept it!  I'll finish it in the next one I make

Gaudiora