I'd like to echo a few things Trebuchet said --

Describe in full detail all little nitpicky 
things about the cel -- including if your scanner 
doesn't pick up certain colors or if the scan is 
cut off a bit (as Treb said, giving the 
measurements of the image can be very nice).

Also, if there is no sketch, then state that it 
doesn't have one. Or if the BG matches or doesn't 
(or even if you don't know). Be sure to comment 
if the cel is stuck to sketch or background.

Lastly, if you are new seller or have low 
feedback on ebay, then your auctions might not 
fetch as high a price as some other well-
known/high feedback sellers. Just something for 
you to be aware of. 

Personally, I'd put up a for sale page with the 
stipulation that you will put them up on ebay 
soon after if they don't sell. But that's just my 
preference ^_^ Good luck!

Wendy (who should be in bed!)