Let me give you some examples of reasons why 
people would do cancel early:

1. About 5 of the bidders have 0 comments. They 
bid on my cel even when the auction reads that 0 
comment bidders must e-mail first. If I cancelled 
the 0 comment bidders, it would plunge the price. 
However, I don't think it's fair for the "real" 
bidder to leave the 0 comments bidders. But... 
how do I know that the "real" bidder isn't 
actually "bid shielding" and that he/she knows I 
will drop the 0 comment bidders (hence ensuring a 
low price)? (I've had this happen a lot with DBZ 
cels). Real bidders are usually scared off by 
lots of 0 comments.

2. If I start a cel at the price I want for it 
(say $200), that's $3.30 in fees even if my 
auction gets no bids. If I start the auction at 
$9.99 but I put a reserve on it of $200 I get 
dinged $3.30   $1.00 reserve fee (NOT refunded if 
the item doesn't sell). I dunno about you, but I 
would rather start the auction at $9.99 and give 
them $0.35 and be able to cancel the auction (if 
below my base price) and only lose $0.35 instead 
of $4.30.

If it did sell for my reserve (say $200), that 
would be $9.43 in eBay fees as opposed to if I 
started it at $9.99 with no reserve fees = $6.48. 
This may not seem like a lot, but if you are 
doing 20 auctions a month with similar results, 
$20.35 in initial non-refundable listing fees is 
a lot different from reserve listing fees at 
$86.00 (if they don't sell) or $66.00 (if they 
all sold).

So, if I see the auction isn't going for what I 
am comfortable with, I have the freedom to cancel 
it and I only lose $0.35. That makes me a lot 
happier than losing $4.30.

3. Sometimes (yes I know this is an eBay evil, 
but it doesn't mean that people don't do it all 
the time) someone will offer to buy out the cel. 
When we know that eBay is going to ding us $12.00 
for a cel if it went for the price that this 
person is offering straight out, you bet people 
will cancel auctions and sell it. It's just good 
business. It's not good for eBay, but it's good 
for the seller. Especially if they know a person 
will pay by Check or Money Order instead of 
PayPal (which adds on another 2.9%   $.35).

4. I've had to sell cels before because I am 
having some sort of financial emergency (who 
hasn't had to do this?) or else I have found a 
cel that I can't live without ^_^; So, I put up a 
couple of treasured cels for sale on eBay. Then 
(this is just hypothetical) my Grandmother gives 
me $1000 as a Christmas present. Whoa! Well, that 
would solve the shortage of funds, so you bet I 
am going to take down those treasured cels and 
keep them as I don't need to sell them.

#4 actually happens very rarely with me ^_^; I 
don't usually put up anything for sale that I 
can't live with selling. However, I know a lot of 
people have a lot less cel stock than I do and 
it's harder for them to choose a cel to sell in 
an emergency. So, I can see people cancelling for 
#4 especially if they only own 12 cels and the 
ones they are selling are their favorite 2.

Anyway, I'm sure there are other reasons...

Just my 2 cents,
Misa