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!)