OK, here's my 2 cents as a relative newbie, but certifiable 
addict....

I really can't say anything about the first question, sorry!

2) I'm not quite sure, but it seems a pretty limited group.
Of all the people I know who are into anime, only 2 others
are cel collectors.  I think most anime fans think we're 
kinda wierd!
3) Its hard to define what the "hot" cels are. If you go by 
price, I think it depends on the art work of a show, and how
short it is (the shorter the series, the less cels there 
are.  Therefore cels from a good short series will be more
in demand because there are less of them to go around)  If 
you go by how many people collect cels from a certain show,
I think it depends on how popular, and to some extent how 
long running, a show is.  Also, the shows that are currently
on Cartoon network - Sailor Moon, Dragonball Z, and Gundam
Wing - tend to be more popular because they have a wider 
audience...not just the die-hard anime fans, but generic
cartoon fans as well.  Tenchi Muyo cels are already very
popular, but will probably become even more so now that its
just starting on Cartoon Network.
4) I agree with what the others have said so far about Ebay.
Because so many people seem to only "shop" there, the prices
go WAY up - MUCH more than they would be from a dealer. I 
recently saw a Sailor Moon cel (FYI, transformation sequence
of Sailor StarFighter) go for over $200 on Ebay, while an
almost identical cel had been sold by a dealer for $77.
6) Cel collecting is an expensive hobby.  You *can* get cels
for $10-20 dollars, but they usually aren't any good (except
maybe to certain people who like them just because they like
them, and not for any of the reasons I listed above).  The
most expensive cel I've ever seen listed was about $1400. It
was from Sailor Moon - a very popular character who is not 
found in very many scenes and is nicely drawn, and more
importantly, it was a cel of her as a child, which is VERY
rare.
7) Personally, the following things determine if I like/want
a cel:
- the show.  So far, I've only bought cels from shows I've 
seen and that I really like.
- personality: I have to like a character's personality
- looks: I have to like the way a character is drawn.  Hair
and eye colors in particular are important to me.  Outfit 
and hairstyle are also important
- pose: I prefer action scenes, and scenes where I can see a
character's clothes.  I also like the character's
personality to show in the cel.  I tend to avoid close-ups.
- additional stuff: I, like everyone, prefer key cels, and I
rarely buy cels without sketches, or where the sketch or 
background is stuck.
8) YES YES YES!!!!!!  I started with one tiny $8 cel, and 
have since spent $1000  in a couple of months. I'm actually
on a cel diet myself right now, and even have to sell some
of my cels, because I just need the money.  However - even
needing money badly, it is a wrench to sell any of my cels,
and I'm not getting rid of as many as I should - and I'm 
still looking at cels for sale!!

Come to think of it, that was probably two dollars worth, 
instead of two cents!!
Az