As to the people I do cels for-

Mostly, I do them for my friends, who like me 
have like no money anyway, and wouldn't sell 
them.  
When it comes to commissions, so far I usually 
get people who have like no money, but would 
really REALLY love a scene of ____.  Of course, 
lots of times it's not a screencapture but an 
original anyway.

Just to assure you, I usually browse anime cels 
for sale on ebay, and if I ever found one that 
was being sold as an official one that was one of 
mine, I'd alert ebay.  First, cuz I'd be pissed 
that someone was scamming someone else, and 
secondly because my cels usually sell for between 
$7-16 on ebay, which is usually 1/30 of what the 
original cel was worth, so I'd be REALLY pissed.

As for the 'it wouldn't match because you'd only 
do what you could see from the screenshot'...well 
that's really not true.  It might not match, but 
the dimensions of a screen capture aren't the 
same as my cels, and I usually expand them out a 
bit to fill up as much of the acetate as 
possible, and I brighten the colors a bit because 
cels are usually a little more vibrant in their 
coloring than they are in the screen capture.  

But I'm going to experiment with signing my cels 
now, thanks to someone's suggestion.  (I don't 
usually sign any of my stuff because I don't have 
the prettiest signature in the world and I think 
it takes away from the overall pic)

^_^ Don't get too worried, I'm starting to think 
some people think I'm going to raid their cel 
galleries and start reproducing them en masse.