I am not a big cel seller on eBay but I have sold 
cels on eBay. This is what I would look for from 
a seller:

-put anime cel somewhere in the title
-use correct punctuation, spelling, and 
paragraphs. (Foreigners who only know some 
English are excluded.) It's really annoying to 
read a mass of words that tries to be English. ^^;
-do not put too many other graphics, fancy 
animation or sounds on your auction page as some 
people still use modems, while others check up on 
auctions at work and do not want to be startled.
-a clear, relatively large scan so I can see the 
details. A fuzzy scan makes me think you are 
trying to hide something.
-giving the measurements of the image is a nice 
add-on.
-be up front about any damages such as paint loss 
or folds in the cel.
-if there is a matching sketch and/or background, 
say so. A picture is nice too.
-if you are selling a non-production cel and know 
it, say so. Being coy by only saying that it is 
an "animation cel" will hurt you in the long run 
as some of your buyers are bound to post in a cel 
forum detailing your exploits.
-don't exaggerate, "THE BEST CEL EVER!" 
or "RARE!" (when I know it's not) makes you sound 
like you're trying to pull one over on me.
-if your scan is zoomed-in to the character 
because it is small relative to the cel, then say 
so. Don't make it seem like it covers almost the 
whole cel when it does not.
-spell out the payment options and the costs as 
clearly and as detailed as you can.

I already decided that I will automatically 
cancel bidders of feeback less than 2, but that's 
about it.

I think this is unfair to those honest people who 
do not have much feedback. I would state that 
they should contact you before bidding, and if 
they do not then you would invalidate their bids.

I suppose this means taking offers instead of 
setting prices, but I would like to know what 
kind of policy works for you the most.

This works for some, but it does turn away people 
who do not want to make offers because they are 
either lazy or do not know what is the right 
offer to make without unintentionally insulting 
you. There is no one perfect method, or else 
everyone would be using it.

---Trebuchet