Good luck with your story.
I figure I'd try to lend a hand as well with my opinions:

Cel collecting never truly "took off" per se, it's always been a niche market at least in the US, mainly because of
the prices associated with domestic animation cels.  I don't see this facet of collecting ever becoming much more
than it already is.  In my experience, most people would not justify spending *any* amount of money on "...a painted piece of plastic..."

As previously stated, Gundam Wing has become "the next big thing" mainly because of its airtime on US TV.  I expect
the same thing to happen to Card Captor Sakura and Tenchi Muyo, as they are just starting their respective runs.  The Vision Escaflowne is sure to follow, once it hits the air...

With regard to eBay, it is a seller's market.  Generally speaking, any semi-popular or rare items that are posted
make a profit.  Sadly enough, items are showing up from dealers before even becoming available for purchase
normally, simply to cash in on the buyer frenzy and generate profits in vast excess to what the actual value
might have been.

Cels can be popular for varying reasons.  It could be the character(s), the overall image, a specific show, the
specific artist that created the cel, or the person behind the show.  For example, there are quite a few people that
collect cels of Ryoko from Tenchi Muyo, others that look for group shots or face-shots that use the entire cel,
while some look for cels from shows like Record of Lodoss War or Bubblegum Crisis, and finally some collect cels from
Miyazaki's shows (Nausicaa, Princess Mononoke, etc.).  There really is no set criteria, actual value is set by the
collector who owns it.
Personally, I collect cels of specific characters from certain shows....

*Anyone* can be a cel collector.  I've only been active for less than 4 years, my collection is small, and only
recently did I pay more than $100US for a single cel.  As someone else stated, there are cels available that won't
break the bank.
The most expensive sale I heard of was at Anime Expo's charity auction.  I don't remember what cel it was, but it
went for over $10k.  There's a posting about it on the forum somewhere....
Personally, my most expensive purchase was just over $300 for a cel and matching backgrouond from Bubblegum Crisis.

My reasons for cel collecting is to own an actual piece of the show--to have some connection with the work that created
something I took pleasure in watching.

Collecting in general can be addicting.  *Anyone* can get caught up in the moment.  Think of it like going to an exciting hockey/baseball/soccer[football] game...